How to regain the trust? DO I walk or stay and try and work it out?

My Boyfriend and I have been together for almost 3 years....I have a daughter from a previous marriage who is 6 and absolutely adores him....I have caught him 2 times before while flirting with other woman on facebook, which totally hurt me but was able to get over with some counselling and decided like a fool to give him another chance....2 years had passed and he recently started a full time night position at a new job, I have met people that he works with including females while dropping him off at work while i was going to work. He added this one girl to facebook that I had met and had conversations with, thinking it was friendly chat. He would chat with her on facebook while i was in the same room but I could never see the conversation, he would be facing me with the laptop in front of him. He started acting differant, he was excited to go to work. I never had his facebook password but I did have his e-mail password....I read a few of the messages from her that seemed innocent. Then a few nights later he tells me he has to go in for 9 instead of 11 because they were behind and it would be overtime. I asked how he found out since they never called he said that this girl told him. I seen a message that gave her address so I got a terrible feeling in my stomach that something was going on. I downloaded a key logger onto the laptop and sure enough the next day when I checked I immediately foundout that he had picked her up early and had oral sex in his car...more may have happened but he denies it all.

I immediately woke him up and confronted him and told him I knew what was going on between him and this girl and he denied it..I told him I had proof....(This lady is 10 years his senior has 3 kids and is in a relationship) I messaged her to let her know I knew what was going on between them. She blocked me from Facebook and was commenting on his posts and status's without me being able to see them. He has deleted her from facebook and says that he had a talk with her.

I have had anxiety over this for the past week, hate that he still works with her becuase I don't know what is happening while he is at work. I know i need to have faith and trust that there is nothing going on between them but find this hard to do. I am hurt, and angry and blame myself. I find myself wanting answers that he won't answer, he tells me if i am going to bring this up forever and I told him it is a slow process and i am allowed to be hurt...

I have been faithful from day one, I was in a 3 year marriage before meeting him. I have told him the truth from day one and has not lied about anything.

I just don't know where to go from here, still very hurt and everytime I close my eyes I can see her there laughing or I see the words that he typed to her...