my boyfriend of 4 years cheated on me for a year and a half

so i(20f)found out 5 months ago that my boyfriend(22m) that i was living with have been texting girls for nudes on snapchat and other apps. broke my heart completely but since i had no other place to go and i was in love with him we've been trying to fix and repair. hes changed completely in this time he treats me better, has been starting his own business and bettering himself and i can really tell he has stopped. he has admitted to everything including childhood trauma that explains a lot. we moved out of his moms house so we can really growup together and got our own place to which he contributes a lot and i really love our life we've been building together. except ive been finding out more information about who he cheated with and to my worst fucking fear it was with 15 year olds not explicitly but they were involved and i wish i could just forgive him since hes been so much better and i really do feel that he will be able to maintain this goodness. but im also terrified that if he can do that then maybe along the years he'll do it again or even worse. i love him so much and cant imagine my life without him but also dont know if im just also a shitty person if i decide to stay with someone who could do that. i can see his perspective for just being so far gone in porn from a young age and using it to cope with trauma but also dont think thats an excuse for doing something so horrible. im so lost because i really just want to stay with him and keep growing like we have been since it has been really good, but am i terrible for giving someone so horrible another chance?