My boyfriend kissed another girl abroad

My boyfriend and I have been together for four years. People doubted we would make it with me being at uni for the past three years but I’m so proud of how we have made it work. My boyfriend went away to Vietnam with a group of friends a short while ago. He originally was supposed to be going for ten days, when I asked what time he was going to be home on the 10th day (Monday) he told me he would be back on the Thursday. I was a little disappointed as I had been counting down the days to seeing him. Tuesday night I called him for a chat and he seemed distant, when I asked why he said he was arranging flights and couldn’t talk. I said surely you’ve booked a flight home already at which point he told me they wanted to extend until Sunday. I was gutted, had been counting down the days for him to come home. I carried on as normal at home, although after that night he seemed distant again over text. I put it down as he didn’t have WiFi and was enjoying himself. Saturday before he’s due home he was texting me things saying I’m amazing and he doesn’t deserve me. The third time he said it I said why are you saying this? Do you have a guilty conscience? He responded it was unfair to tell me over message and boarded his flight. My anxiety was so high I had to go home from work I was so wracked with worry over what he had done. I couldn’t eat or sleep. When his flight finally landed at 6am I was still awake and he text me he had kissed a girl ‘unemotionally’ one night. He seemed very regretful and told me it didn’t change his feelings towards me at all, he realised what a mistake he had made and was sick the rest of his holiday after it had happened. The girl kissed him, knowing he had a girlfriend, he admitted he kissed back in the club and then stopped it. He said he went to the toilets got upset then headed home. It’s been a month since this happened and I still think about it when I’m on my own. I want more than anything for things to be as they were before he left for this holiday. I was stupid enough to search the girl online and she is attractive which has made me have really low self esteem. My family and friends say I need to leave him and go out and sleep around but that’s not what I want.

Constructive advice welcomed.

Thanks for reading.