Hey Cathy,
I'm not actually diagnosed borderline; I post here because it's one of the few forums I feel comfortable in. But it sounds like you're in a lot of pain and you haven't gotten many replies so I hope it's okay if I respond. I think if you're hurting badly it's always good to reach out to people; it doesn't matter if you're a teenager and being unhappy is "normal." Asking for help is difficult so I think you should be proud of yourself for that.
Unfortunately, I can't diagnose you with anything 'cause I'm not a therapist. That said, I completely relate to what you described, the fear of being abandoned. [Actually, you remind me a lot of me, especially when I was 18, so apologies if this is really long. These are things I still need to remind myself of.

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Maybe you could slow down and try to address the specific causes of your anxiety. For example, you said that when you're in relationships you have trouble trusting that your SO will not leave you. You also said that you need a lot of reassurance from the people you're in relationships with, whether it's a romantic partner or a friend. So here are some questions for you:
1) Is this something you feel with everyone all the time, or do you find there are certain triggers that set it off? Do you find that your own self-esteem plays a part? I can say that working on my own self-confidence has helped ease my fears a LOT. My relationship brings me more happiness than anything else my life, but I actually think that both of us getting involved in things outside of it has made it stronger. Also, making my SO my only source of self-worth puts a lot of pressure and responsibility on him and I don't want to do that.
Maybe pursuing some other hobbies and connecting or re-connecting with other people would make your life feel more balanced and stable? If you feel like your BF is the only thing you've got, of course you're going to be terrified of losing him.
2) How do you cope with fears like this when they come? What ways are helpful and what ways are not helpful?For example, I've gotten markedly better at trusting that my SO loves and cares for me, even though he's kinda avoidant and has difficulty showing it. I still struggle with it tho'. This is a small thing but last weekend we were supposed to hang out and he never called me. He's someone who needs a lot of space and alone time, so I made a point only to call him once.
But -- I can't lie -- it hurt! I spent Friday night and Saturday by myself; that wasn't healthy and only made things worse. Then, Sunday, a friend of mine invited me out to lunch. Getting out of the house and seeing someone outside the situation helped me feel better and get some perspective. I remembered that he and I hadn't had concrete plans, that there are many reasons he might not have called, etc. There was no reason to leap to the conclusion that he didn't like me any more.
If you're anything like me, it's not that there aren't other people you like or other things you enjoy doing. What makes you upset is not that your friend can't hang out with you; it's that when they
don't want to hang out with you, you imagine it means they don't care about you and are going to leave.
3) When you feel like you're not getting enough reassurance from someone, how do you handle it? Do you tell them how you're feeling? I've noticed that people with personality disorders often struggle with this. Even those that are naturally empathetic can be blinded by their own pain and fear, and will forget to consider the weather in other peoples' heads. Unfortunately, communication is probably the most important thing in any kind of relationship. One thing I always try to ask myself when I'm upset is: "What does he actually know? I know what
I know, but what have I actually told
him? He can't read my mind: have I communicated my feelings clearly? Or am I expecting him to pick up on passive-aggressive hints and subtle signals?"
You mentioned that sometimes the things you do to keep people close makes things worse -- that's a really good thing to be aware of, and it's something you can change. When you feel rejected by someone, are you up front about it, or do you just play it off like you don't care? Do you bury your resentment so it explodes later over something small and random? Are you vaguely cold and insulting without ever telling them why they're being punished? Do you vanish or give them the silent treatment? Please believe me, if you do, I'm not criticizing you: lots of people, disordered and non-disordered, handle pain in these ways. Unfortunately, these ways are really ineffective.

I know it can be seriously hard to tell someone who has hurt you "You just hurt me." It feels like giving them even more power. The thing is, unless he's a closet psychopath, your SO seriously does not want to hurt you and does not enjoy having power over you. I know it can be hard to trust this if you've had bad experiences with people, but if a person loves you, they
genuinely want to see you happy and well. They don't enjoy making you feel bad. They don't look down on you. Honestly, if your BF hurts your feelings, chances are it was completely unintentional. If you don't say anything, he's gonna keep doing it because he's not telepathic and doesn't know it's causing you pain.
Can you work on expressing your feelings in a calm and straightforward way? "It hurts me when you talk about how amazing your exes were because I feel you're comparing me to them" or "I'd feel more comfortable that you care about me if you wrote or called me occasionally throughout the day." You might be surprised to find that your SO will be more than happy to accommodate your needs once he knows what they are. He may be touched to learn that you care about him that much.
The other thing I would add is that everyone -- everyone! -- makes mistakes in relationships. No one is perfect, and that's okay. The people that care about you do not expect you to be perfect. They will understand and forgive. And I guarantee that none of the people you've been involved with were saints either. You can do them a favor by being understanding and forgiving yourself.
As for dating someone borderline, I don't know. On one hand she may be more understanding of your issues; on the other, she may have attachment issues of her own. Not sure about that.
Last thing: what you said about wanting to end things. I get that. I really, seriously do know how that feels. All I can tell you is that if you don't give up, one day you will look back and be so, so glad. Life changes in all kinds of unexpected ways. You'll meet people who will teach you things you never expected. There's so much out there you haven't seen and can't imagine. It sounds like you are very sensitive and very loving, and that is a rare and beautiful thing! Unfortunately, sensitive and loving people endure a lot of pain, but they also experience great joy. If you can stay strong and trust in yourself, it will be worth it.
I wish you luck!