Overcoming insecurities

Hi guys! This might be long but I’ve been with my man for a year and a half. When we first started talking and before we made things “official” (even though we made clear we weren’t seeing anyone else) I was a bad human being and snooped through some of his messages and found he was still in touch with his prior ex that he was with for a year or so.. from what I’m told it was toxic but I know she was his first love.. ever since then I’ve felt insecure about myself and his feelings towards her however I confronted him about it and he parted ways. Fast forward to a year later, I see that she messaged him telling him she wanted to apologize.. and he never responded. This was 2 weeks ago. I’m happy he didn’t respond but now I’m back to feeling so insecure and worried he will end up responding. Idk I just can’t get out of my head that he still loves her and misses her even though he didn’t reply.. am I being flat out stupid or how do I get over this hump?