It's not you. The trust between you has been broken and he seems unwilling to help repair it. I too am a student and my professor recently gave us these guidelines to follow which I will print here. Sadly, part of you will always wonder with this man and is that how you want to live your life? My ex had an affair and turned out to be a liar of the worst kind. The opposite of responsibility is blame. Do you have children with this person? He doesn't seem to care about repairing the damage he has done. You may need to cut him loose and move on. I don't know that he can be trusted and you deserve to be happy.
DR. LEFEVER'S GROUND RULES FOR OPPOSITE SEX FRIENDSHIPS
1. No opposite sex friendships that do not involve your partner.
2. Your partner is present during friendship activities more often than not.
3. Your partner knows each and every time you spend time with an opposite sex friend. This includes when, where, how long, and for what purpose.
4. No meeting with opposite sex friends behind closed doors or in secluded areas.
5. No meeting with opposite sex friends in any way that would cause a reasonable outsider to wonder about the nature of the relationship. This means, no late night get-togethers or phone calls, no innocent overnight stays, etc., without your partner present and in full agreement.
6. No private or secret talks with opposite sex friends, and no discussing personal topics without full knowledge and agreement of your partner. You don’t keep anything you discuss from your partner.
7. No working behind closed doors, sharing an office, or spending casual time at work with colleagues of the opposite sex without the full knowledge and agreement by your partner. This includes “only” going to coffee or lunch together. Fully disclose and discuss such activities and be completely forthcoming with details.
8. No flirtations, no matter how “harmless” you think they are.
9. No opposite sex friendships in which you invest more time than you do in your relationship with your partner.
10. No behavior that puts loyalty to your friend above loyalty to your partner.
11. It is not up to your partner to “get over” apprehensions or other emotions caused by your behavior—it is up to you to get over the behavior. It is up to you to change your behaviors in a way that makes your priorities clear and puts your partner first.
Share these with your spouse. If he is unwilling to follow these guidelines then you have your answer. Good luck in whatever you decide to do.