Hey Risa, opening up to your boyfriend can be pretty tough. And it doesn't help to have his ex around. But without communication and integrity, no relationship will survive. So don't procrastinate until the opportunity is lifeless, go for it! If this relationship means anything to him, he will love the fact that you have opened your heart. More than anything, he will understand to make the effort to put the conversation into full effect. And as far as the ex, you should also have a chat with her, and let her know that you and your boyfriend (her ex) have a relationship that could potentially increase. I call that the "For the Record" speech...no matter how you say it, you're setting the record straight. About the "freezing-up"... When you freeze-up, your body is self-consciously letting you know that tension could arouse, and that you need to prepare yourself for any sort of reaction(s) that may occur, But it's no biggie! You'll thank yourself when you get it over with!
P.S. Not to blow your cover Risa...but you should let your boyfriend know that you had a talk with his ex, just in case she's the "go back and tell him about it" type. That way, there will be no warning alerts, or arguments! But this should be the easiest part, not a sweat!!!