Are you unhappy in your current relationship?
If you really do love him and do not want to blow up your life: put more effort into your relationship and block this ex. Stop it with the ex. Get horny for your fiancé.
If you’re unhappy, the ex is a distraction that’s telling you you are not happy, and you need to work on the relationship or leave it.
I would really encourage therapy and some deep reflection on your current fiancé and what you actually want. If you’re engaged saying you went into this too fast that to me is a red flag that you aren’t happy. Too fast isn’t too fast if it’s right. The ex is the canary in the coalmine.
Or he’s not. You get to choose what happens next. Be wise.