LOL

I agree with turnera that if your counselor was given exactly what we've been given and still said "get over it" then that is not good advice. I have to admit, I do wonder if there weren't other things or facts that the counselor has though--or maybe the other side of the story?
However, here's one thing that may be good, practical advice to help you explain and envision it better. At this point, you trust something when the words and the actions match. Your husband words (it started in May) and actions (see the cell phone bill) do no match and thus it is reasonable to not trust. Furthermore, you did trust him and he betrayed that trust, so again it is reasonable to not trust.
(FYI, if I were in that exact position and someone betrayed my trust and then said to me "You just have to trust me" I'd say "No, I can also choose to NOT trust you unless and until you take the time to earn my trust.")
Anyway, try to envision a puzzle with a zillion pieces. You have been given some of the puzzle pieces but no matter how you try to put the pieces together, they don't make a full picture that you can see. Your spouse on the other hand has all the pieces and sees the picture. Now the picture may be less than flattering so they don't WANT you to see the picture--nonetheless they hand you a few puzzle pieces that they know don't really show anything. Since they won't show you all the pieces, and you can't put it together, the puzzle drives you NUTS and you think about it all the time!
I would suggest that you talk to your spouse and explain to him this puzzle analogy. Then let him know that you realize at times it may feel like an interrogation to him, and let him know that you will not hold the Spanish Inquisition. Then see if you can't agree to asking one question a day (or maybe two) and see if he'll agree to answering one question a day honestly and thoroughly. That way, gradually, you will begin to get all the puzzle pieces and it will be a little after the initial discovery when it's so painful. Also that way he will gradually realize he can be honest with you without being screamed at, and he will not be tortured endlessly for more info!
Finally at some point you are going to have to make the decision to say, "Okay that's enough. I know enough now that I'm making the deliberate choice to let it go and I'm done. There will be no more." One thing that is vital to your recovery will be that your husband has to have some hope it will not be held over his head forever. If he thinks you will just obsess about it forever and "make him pay" then he will withhold or even give up. Okay? Make sense?