you are with him 18m and think he's a genuine guy?
you don't know him at all. he was with his ex for 13 years and she didn't know him at all either.
living with a bloke like him will drive you batshit. perhaps he is the source of her issues, but he keeps feeding her just enough to keep her on the hook.
he's already broken the cheating taboo, if he'll cheat on a woman after 13 years, (that he's admitted to) how easy do you think he'll find it to fuck around on you?
this is a dealbreaker, you can tell him to stop her contacting you as it's disrespectful and not supportive of his choices for a start, and to sit there sending 'it'll never be ok' messages is out of order.
did he send back 'I miss you too?' that's an immediate sacking tbh, she's crossed the line, but he did too if he sent that back to her.
he's not nearly scared enough of losing you. he knows you (and all women for that matter) will just keep on taking it, if they don't, he'll get another.
mark my words, this idiot will start playing her off against you.
my ex did this, for 10 years I put up with her tearful international calls. he knew it upset me that he didn't tell her to back off, despite living together, having a child etc, but he exploited that to manipulate me.
this man is not right to date. end it (it'll shock him to his very core) tell him there are 2 in the relationship or none at all and to not bother to contact you until she's been put well and truly in the past.
if he does it, great hurdle overcome (not the end of the story by a long shot I fear, but hey) she will be gone, you will have 'won' and be free to enjoy the relationship you are hoping for.
if he doesn't, then you haven't wasted any more of your time on a feckless, cheating twat who has no respect for himself or women.
win/win i'd say.