my partner and his ex...

my partner and i are engaged to be married. we had been so happy and in love. we both thought he was over his past heartbreak from his longterm relationship before me until recently. he knows i had been insecure about being the “new girlfriend” while he had years worth of love and memories with his ex. even now, we’ve only been together almost 2 years, but he said he hadn’t felt this way about anyone before and he proposed to me. he’s over his ex, i know that. he’s happy and in love with me, i know it and feel it. his ex reached out to him yesterday about a mutual friend gone missing with car found abandoned on the side of the road. that opened a conversation for them for the first time since they broke up. she asked him to meet up and catch up and he said no because he didn’t see when there would be a good time to meet (he wouldn’t want to leave me home on the weekend while he went to catch up with his ex). however, he told me he was sad and emotionally triggered by that painful breakup. that hurt me! if he is over her, shouldn’t he not be able to be triggered anymore? he explained that he’s over her but maybe the pain from that time wasn’t completely healed and he hadn’t realized it until he heard from her for the first time last night. now i’m upset that she can still trigger emotions in him like that when he and i are about to marry (and i’m 6 weeks pregnant). i get that he doesn’t wish to be with her and doesn’t care to hang out with her anymore but it bothers me and feels so unsettling that her breaking his heart could trigger negative emotions in him last night and if he saw her he says he thinks he’d be reminded of the pain. she proposed to him back then after 6 years then decided months later she wasn’t in love with him anymore and she left him. he was broken from that then... i just would like for him to feel nothing towards it now but that’s not the case and i just have to accept it...