You came to the right place. I'm sorry you are going through this. Definitely read, here and elsewhere. Cynical One's blogs are excellent. Melody Beattie's books, among others. Maybe a support group - Nar Anon or the like.
You are not stupid. Please don't bash yourself for believing in someone you love or for not seeing something that you never expected. I KNEW my husband was a recovering alcoholic and I didn't pick up on things as he worked his way to relapse. It happens. I feel for you, the dishonesty is hard to take. As mfox said - take care of yourself above all else. Keep coming here - the support and understanding is here for the taking.