Need some help on infidelity?

My husband and I are separated but working on reconciling. I am positive he slept with a bar tramp a week or so ago. He suddenly is fully commited to me, he says, wants me to visit him as often as possiable, yet he mentioned this women several times during my last visit. Why would he sleep with someone and then tell me he wants to spend the rest of his life with me, also tell me he loves me more then I could imagine.....then again he also said this before sleeping with her. We have been married a long time so I hate to walk away if he is truly in love with me but, given what he did how do I know this.