Just need some genuine advice....kinda long

Let me start by saying I've been with my SO for 13 years and we have had our ups and downs. We have 2 daughters together and everything was going great. We decided to ttc our 3rd child a little over a year ago after very long talks about how scared I was to get pregnant again because of him leaving me for a couple months at 8 months with our second. So anyway I finally decided to put the past behind us and move on to ttc. 6 months in to ttc I found out he was talking to a girl through text msgs that he worked with and I was absolutely devastated but decided to move on and still ttc he promised it was "nothing" and he would never do anything again well i ended up getting pregnant in Dec and had a miscarriage 😞 and to be honest I was kinda ok with that since i wasn't sure how I felt anymore but now it's all weighing in on me and I'm just not happy in my relationship he is a great guy he works hard and he is a great father but I'm just not happy anymore and I'm not sure what to do 💔 i have been with him since i was 16 and I'm afraid that if i move on I will regret it. But I also dont want to be unhappy by staying. I am so lost. I dont want to leave him and the kids be affected by it or him. I tell him I'm not happy and he just says if u leave him he will kill himself or go off the deep end he says he cant live without me and the kids were all he has so that makes me feel horrible! Sorry it's so long i just need some advice.