For any relationship to maintain , communication is the key. I would suggest that you both should sit and communicate and discuss this matter seriously. discuss each other's expectations. See what is lacking. It is very important that you should have respect , trust in your relationship. Rebuilling trust can be challenging for both of you.
Next Steps
See a couple therapist / counsellor , If he is promising to be faithful to you, henceforth, let him be transparent in his activities..time and again he must share his mails,messages etc to rebuild the trust.
Health Tips
you must take care of yourself, it is very important that you should be calm,and patient . if he is really feeling sorry for he has done ,learn to forgive to maitain the relationship. .This will take time.