How do u move past infidelity?

on tuesday i caught my fiance cheating on me. i've decided to take him back but how do we even begain to get past this. he doesn't have a clear answer as to why he did it. he's been depressed for about 3 months now and i think he needs to speak with someone about this. it's harder for me to trust him and he hasn't left my side since this has happened and even when he does leave he is always calling me letting me know where he is. i just don't know how to deal with this b/c i am scared that it may happen again and he is really show his emotions now and he's cring all the time ( so am i ). and forever saying sorry. i want to believe that it will never happen again. he says that he realized that he was taking me for granted and that i have always been so good too him. i want to know why don't people realize things before they do it. How do u move past infidelity?
I personally couldn't, Once the trust is broken there will always be questions in the back of ur mind.......