He started talking to the girl he used to love again. What should I do?

First, before you do anything, make sure you're very clear about what's happening with the two of them and if it's actually something you need to worry about, at all. Make sure it's not just a fear or insecurity you have, that's being blown up in your own mind.

If you're sure he has very strong feelings for this other girl and would be with her if he could then you need to think about this a certain way. You know how he feels about this other girl? Well that's how someone should feel about you - you deserve that. You want to play the role of the leading lady in your own love life, not the friend, or the rebound, or the temporary girlfriend until he can be with the one he loves.

No one wants to feel they're playing second fiddle to someone else, so don't. You're right that it sucks, and it will for a while, but you have the ability to change all of it.

You're not meant to be alone.. you're just hurt right now. You can't make him love you more, but you can respect yourself. If he hasn't made it official, he's not really committed. 6 months is a long time not to refer to someone as your bf/gf. Whether he ends up with this other girl or not is irrelevant. You don't feel you have all of him because you don't feel he's all in and now you feel he has strong feelings for this other girl. She doesn't matter - you do. If this is not what you want then you can take control, don't wait for him to determine what you get until he decides he wants to be with her (or someone else).

Here's what you can say:

"There are really great things about what we have, but I realize I need something different. I think it's time for me to move on." Notice how I didn't say "it's time for us to move on." This will get his attention. He may say something that will surprise you. It will certainly have him consider your value and think about what he may be losing. But, I don't suggest you do it as a bluff. I believe you have to mean it. The fact is, you would mean it. To think he has stronger feelings for another girl he's always been into would be torture. Now, please reread my first paragraph.

if you arent a couple how have you been dating for 6 months? sounds like hes a friend and you think you have a boyfriend......