My ex boyfriend 21M and | 21F broke up after 2 years and I have a lot of built up resentment? Help

We both got together at 18. Our relationship was very up and down but we had so much love for each other. I would sometimes see texts from a girl in his phone. He would say "it wasn't like that" then end up apologizing for texting her and he seemed very genuine with his apology. He would then spend the next 6 months regaining back my trust, proving his love for me, showering me in love, with dates, quality time, allowing me to check his phone, and genuinely becoming the best boyfriend ever.. until he texted a girl again. This happened 3 times in total. This genuinely confused me because our relationship would be so amazing and then he would go text a girl again, it didn't go past texts and the conversations were very short so I excused it (it was also my 1st real relationship). It's so confusing to have so many good memories and moments with someone and then have them betray your trust like that. This is someone who i unfortunately have to see often and I don't know how to navigate the situation because after reflecting on it, I'm still very hurt. I really did value and cherish all the time that we had together and I still have a lot of love for him but I just cannot believe that he put me through that 3 times. Any advice?