I feel somewhat betrayed?

So my boyfriend has/gets a lot of tattoos, and a while back when we first started dating he allowed his female best friend to come to one of his appointments. (This is the same female friend from my other posts). I had never been to one before, but I really had wanted to go. I wasn't able to because of school. It really bothered me that he asked her to come with, especially since that appointment was the very start of a tattoo. It felt like since I couldn't go, she was the next best thing, as though she was replacing me. (they also went to get a piercing of hers after his appointment). I also felt like that's the kind of thing that he and I should do together as a couple, not her and him. I felt left out. He didn't see it as a big deal to have her come with, but I expressed how I felt and he understood after a while, and we were all good.
But now, I find out via Twitter (kind of stupid I know) that he ASKED her to come to his next appointment. And it looks like it's going to happen since she's free.
I feel somewhat betrayed. He knows very well I can't stand her (for many reasons) and that I feel extremely bothered that she's going with him to it, yet he goes and ASKS her to come...
How do I go about handling this? I don't want to seem stalker-ish and clingy, and tell him I saw their conversation.

Betrayal? That's what you get for stalking...

Okay... I remember being in your situation, don't get to a point where you invade their space. If they don't invite you, ask maybe once but be prepared to be let down, as they would have invited you along from the get go.

So don't jerk yourself around.

The best advice, which also turned out to be some of the worst advice ever, came from this homeless guy. I was all up in arms and running around frantically looking for someone who was previously living with me. They had taken off to the park to hang out with dog collar fetish types, and enjoyed hanging out in the grass on blankets ect...

They said... don't chase them... well... it's the internet, so chase them all you want otherwise they'd use a fake name, or they'd spread their presence out to multiple places to dilute the stalker behavior ect...

So... in hindsight... let her know it hurts you... but don't get vengeful about it. Be really really careful not to get vengeful about it.

Hope that helps...

I don't want to seem stalker-ish and clingy, and tell him I saw their conversation.

Yeah, at the same time you don't want to miss out on something. So make the small gesture to let them know you've been slightly hurt... don't get all jealous and crazy... one comment over the phone is basically it and then play it by ear.

I am not a suave romantic type, so this advice is probably not the best.

I might have stated this before, but I think you're insecurities are actually well-grounded which isn't the case all the time. He's a little bit more intimate with this female friend than I would prefer as his girlfriend.

I'm not suggesting that you should break up with or that I'd be right in doing so, but I'd consider breaking up with him if I was in your situation. Again I'm not saying I'm right or wrong, but that's what I'd do because I don't know that I'd want to deal with that any longer.

I'm in the same camp as Kabuhi on this one.

I find out via Twitter (kind of stupid I know) that he ASKED her to come to his next appointment.

If I understand you correctly, it is the quote above that troubles me - he lied and told you he wouldn't go to the appointment with the other girl, but then invited her anyway.
If he just told you flat out that he was going with her, that's one thing, but the sneaking around behind your back is more of a concern.

Personally, I'd dump him. But that's just me.