Advice please

Please only give advice if you have truly loved a man more than yourself.I know how it sounds but its the truth,me and my partner both 19 have been together since we are 16 and have been arguing the last few weeks,he calls me and randomly tells me that he cheated on me 2 years ago (a couple months into the relationship) with 3 different women in the space of a week.He claims its because he never thought that we would get so serious which ai understand as neither did I (although I stayed faithful?) I always had a gut feeling since his boys trip was suspicious and I was being ignored for the most part of it,and he admitted that he doenst regret it as it made him “love me more” but also “start to resent me” for months after it and told me today that he feels a weight has been lifted off of his shoulders but I just feel so used.I have been trying to look past it seeing as it was years ago but I have asked him many times to his face if he has ever cheated as I always had a feeling and he told me no and I am more upset over that I think.I always put this man before me in every which way and I truly don’t want to lose him but I feel like I need to leave him for the 16 year old girl that put her trust in someone and truly believed that he would never do this only to find out that he was never who I thought.Only last night we stayed together and had a great time and I really don’t know what to do??