PLEASE HELP? I"M SUFFERING! What should I do ?!?!?!?

My boyfriend and I used to love each other very very much. But these days I realize that he is acting very cold to me, and he acting a little intimate with the other girl (who likes him). I know that he's being intimate to this girl because they need to practice dancing together for their performance. They seem have some of very intimate act during their performance so I'm VERY worry about that. He, once, never talk a lot to this girl. Now he's very open, he talks and smiles with this girl A LOT, and often look at her. I think he's losing interest in me and started liking this girl.
NOW, the really BIG problem is that this girl likes him. Today after work they need to go to practice dancing together at a private place so I'm EXTREMELY worry now! She likes him so OF COURSE she gotta do something to flirt him at that private place and I realize that he's also a little like her too so what can I do now? (I can't go to that place for some reasons please don't ask about that reason).
And ALSO in the next Monday after performance with that girl at the morning, I'm going to meet him at the evening. What should I do for him to forget her? What should I do if he's being cold to me? What should I do if they're being so intimate? PLEASE HELP! I'm very suffer right now.
PS: I love him VERY MUCH so don't ever advice to break up.
Thanks for reading.

You can't win someone over if they just simply lose interest. And you shouldn't have to. Even though you said you don't want to, break up. You shouldn't have to worry about other women in his life...and if he's straying then there is nothing you can do but to let him go. Trying to smother him will push him away even farther.

You need to talk to him, get everything out in the open and find out why he's been cold towards you.
He may well have lost interest in you and developed feelings for this other girl, in which case breaking up with him and going no contact is all you can do.

you need to say something... you also need to trust him unless he has given you reason not too.