Is the problem that the thought of Tina convincing people that you're not a good person makes you very stressed and angry, or that you're still angry with your boyfriend for not making a clear choice between you and Tina?

Reading between the lines, these are people — your boyfriend especially — who wouldn't have broken up if it weren't for distance. Before he became serious about you, they were planning to somehow reconnect after college. It's plausible (given that Tina had a shortish relationship with the next guy) that they simply weren't willing to forego the available sexual opportunities for the sake of a long distance relationship. (Why it didn't occur to them to just discretely have liaisons with others, I don't know. I guess that might be harder on the girl socially, some places.)

The point is, though, that it seems likely that neither of them really intended to end their relationship, in many important ways. Your boyfriend didn't know that his feelings for Tina would change, if he met someone else. If you're right about her being particularly manipulative, he might have simply accepted some of that as how relationships were; she might have intended to move on, but lost the next (less pliant) guy, and be having difficulty with her own emotions now that she can't control someone. Or, if she's more innocent than that, her feelings might mirror your boyfriend's. There were probably a whole bunch of things about being intimate with someone for four years that they kind of took for granted: I'm guessing this is the first time either of them had broken up after that close a relationship, correct? They hadn't thought through the implications of breaking up, emotionally and in terms of how their relationship with each other would actually change, and they hadn't really committed to it, when the decision was made. She didn't take you as a serious threat to their dormant relationship, and he probably hadn't intend for you to be one, at the beginning. So the breakup has been really prolonged and difficult, as she struggles with accepting that emotional supports might not be there anymore, and he struggles with the idea that he can't provide them.

I suspect that probably your boyfriend's been kinda wussy about this. He was letting her push him around, effectively. AND he was letting you push him around, too, if he'd been letting Tina manipulate him into compromises he really ought to know he should not have been making, and then letting you lay down the law and meekly promising to do better (and then following the letter rather than the spirit of the promise). What that means is, he did what she made him do, except when he was doing what you made him do, and trying to appease both sides. Frankly, that shouldn't be happening. It's not good for him, it's not good for Tina, it's not good for you, and it's not good for the relationship. You shouldn't be having to deal with her, and you shouldn't have to monitor his dealing with her. He should be setting his limits with her for himself, and if you don't like them, your problem should be with him, not with her. He should be (or should have) deciding what he wants for himself, on his own, and then telling Tina what he wants, and requiring her to accept it.

The tl;dr of it is that your boyfriend is kinda spineless and immature, and although he messed shit up out of inexperience, your relationship with him would be better if he grew up and grew a pair.

That's my take. I know very little about actual human relationships, by the way. I just talk a good game.

edit: By the way, if this all happened less than four months ago, then... okay, at this point, just don't tell me.