Okay, like I said, I was in a very similar situation. Let me tell you how it turned out....
I was the main earner in my marriage with an extremely caring, loving man, who I'd been with for 15 years. No chemistry on either side for about 10 of those years. But I could imagine being without him, we were best friends.
Started working closely with a colleague who was recently seperated, massive chemistry on every level, we just "got" each other.
We went away on a business trip, I was sure that there was no way I'd cheat on my husband...... I did.
My colleague and I have now been married 3 years and are incredibly happy, my ex husband has accepted the situation well (he knew our marriage wasn't right) but I was in absolute hell for years because of the guilt I felt, My ex is a lovely guy and didn't deserve to be cheated on.
What I would do differently would be to leave my ex before I started a relationship (I did within a month, but I still cheated).
If you think you can handle it and get over what you think is a crush then keep going. But please don't be alone with him, go for a drink and most of all could you tell your husband you have a stupid crush - that would knock it on the head straight away.
Good luck!