Do i try to forgive him?

i have been with my fiancee for 3 yrs, we r supposed to be getting married in august, but for about 2 yrs we have had no life, down to him not me, i have been really understanding about this and have tried to talk to him, but he just says he loves me, and still finds me attractive and dont know why, anyway, recently we have had a disagreement at home, as he was being kranky with one of my kids, but i thought we had sorted this out, he has always been very loving towards me, and affectionate, last weekend i went on a rare night out on my own with friends and he done the same, we usually do things together, he didnt come home till the next afternoon, and told me that he had gone home with a couple he met in the pub to carry on drinking, as this was out of character i didnt really believe him, but left it alone, we chatted and he told me he loves me, and wants to spend his life with me, today i looked at his phone for the first time and found messages from some one he obviously met the other night, i confronted him and he addmitted he had met and slept with a woman from the pub, he said how sorry he was and how he regretted it etc, but how come we have no life yet he can do it with someone else, my heart feels like its torn in a million pieces i really love this man, and dont know wot to do, how could he hurt me like this, we are not kids we are both 40 yrs old, can a relationship stand betrayal, this is making me feel physically ill,