Help understanding relationship with Histronic

I'll try and keep this as short as possible. I started dating a girl I'd known since high school towards the end of last year. We only casually knew one another and didn't keep in contact over the years and are both now 35. She'd always been a girl who was super outgoing, nice and went to Church all the time. I went into the relationship giving 100% of my trust.
During the relationship she was always nice and caring to me. She'd cook me dinner, care about my feelings, ask me about my day, check on my parents. If I was hard on myself, she'd built me up. She'd always try and help me if I was having issues etc. She never once tore me down or was mean to me in anyway. She'd also always be willing to pay for things etc. and not ask me to do so. Of all the girls I'd ever dating she came across as the most caring and kind.
One of the first times we were together we were talking about past relationships. She had actually dated someone I knew of but not well and I was kind of surprised she had dated him. She told me all of her past relationships had been abusive and had been cheated on which was how mine were. I felt like I'd finally found someone who I could relate to and that knew my pain.
There's more I could add but to save time I'll get to the point.

Towards the end of last month, she had told me she wasn't feeling well. This wasn't too uncommon as she seemed to always be having some sort of health issue. I sent her some flowers to make her feel better. However, over the weekend and into the following week she was basically not even responding to my texts or calls. I wrote it off initially because I figured she just felt really bad. However, going into the next week she still wasn't really responding and so I started having thoughts that perhaps she was seeing someone else or that we were going to breakup. To add, during the relationship several times she brought up how it bugged her that I wasn't a man of faith so she couldn't talk to me about Church etc as I couldn't relate. She would also get frustrated at times saying I never tell her I feel about her and when I'd ask what she meant by that she'd say just tell me anything!! Never being specfic on what she wanted. Now, I kind of feel she was just wanting me to tell her I loved her but I just wasn't there yet. However, when I would give her compliments or tell her she was pretty etc she'd be like you don't mean that or be dismissive or say DO YOU REALLY MEAN THAT? It's like she couldn't just take a compliment.

Anyways, she finally texted me the next Friday and asked if she could call me. She called and I could tell where it was going. We spoke for about an hour and she basically said again she'd like to find a man of faith. I told her that if that's what she wanted then that's what she deserved. She was crying and everything but for some reason I wasn't feeling a ton of emotion over it. I think for once I was happy that I had a relationship that was at least honest and respectful with no lying or cheating. And I was accepting that we just weren't for one another but at least I knew that I could move forward and find someone that would treat me right.

We didn't speak anymore after that phone call until about a week later when I see her post on FB a picture of her and another guy. Not only is she in a relationship with him but their engaged!!
I'm in total disbelief. I sent her a message asking her why she couldn't have just been straight forward with me. She responds saying she knows this doesn't make any sense but that they'd known each other for 10 years and had hung out after we broke up and he confessed his feelings to her. She apologizes a few more times during text but then says she's not going to keep apologizing as we are broken up etc.
I'm feeling all sorts of emotions and I decide to look into the guy and end up posting a comment on a picture of her and him and say "Funny I was just in a relationship with her less than a week ago" She calls me the next day angry that I'd post that and that it was none of my business etc. I told her that she had emotionally cheated on me at the very least which she kept denying over and over. Finally admitting that yes, they spoke while we were together but that it was never anything "like that" And continued to say they first hung out after we broke up. She even said that if I would have called her after we broke up that maybe this wouldn't have even happened. Which as you'll see below is complete bs with what was going on already.

THEN... two days later I get a message from her fiance. He's asking for proof our relationship as she told him we were NEVER together and that we had talked a long time ago and that was it. I'm in total disbelief again as she's telling someone I was never a part of her life or anything. I proceed to show him a bunch of text messages, pictures etc. He's in disbelief himself saying he had already caught her in a couple of lies but brushed them off. That he couldn't marry someone that had lied and cheated on him etc. He then said he was going to have to pray about what to do etc. And then a day or two later he kind of changed his tune and blocked me and they're still seeing one another to my knowledge. I will say I was suicidal for a few days as I totally lost who I was over this.
Mind you we also had mutual friends who knew we were dating and one of her old best friends actually dates my brother. Her fiance also told me they'd been talking since August 7th (we split on Aug 25th) and there was even a picture of them on August 15th at her house embraced in a hug on her couch.
After talking with my brother's girlfriend, she basically told me that she'd cheated in the past, seemed to always be overdramatic to get attention. And basically, they'd stop being friends as she felt she was fake. And that she always thought she'd found the one with every guy she was dating for the most part. She didn't tell me upfront as it'd been a long time and thought maybe she had changed. She has no long-term friends at all and seems to only have one female friend currently. If there are any long term friendships it's with guys.
I then talked to my friend whose buddy had dated her a long time ago and questioned him with some of the things she had said. I found out that the guy she was seeing came home one day to her hooking up with his roommate and then started dating him. She actually got pregnant from him, and they had got engaged but she had a still born.
Looking back now I see other lies and story's that she made up or that were half-truths. I also realized that anytime we were out somewhere, and the attention wasn't on her then she'd seem to always end up sick and I'd of course give her attention and care for her and most of the time we'd leave. Or if it wasn't her being sick, she'd tell me sexual stuff she wanted to do which now I feel was just to get attention back on her. One of my friends who only met her 2-3 times actually told me afterwards that he felt she had mental issues. When I pressed him about why, he said that he felt she was faking stuff to get us to leave places.

Anyways, it's just hard for me to wrap my head around how someone can be so nice and caring as you'd figure if they had a disorder, you'd see some kind of sign or they'd be mean to you in some way. Or that they'd be wanting to use you for something like money etc.
Also, why would a histrionic woman want to get married? It seems as though it would make it tougher for them to talk to other men etc. and easier to get caught. Talking to her old best friend she did agree this is the first guy she's dated that seems to really be showering her in affection etc.


I'm just looking for any insight on understanding this all a little more.