Welcome. Prayers for your healing!
Before you begin, yes stop having sex outside of marriage. This one is a deal killer. If your boyfriend/fiancee will not respect your faith, then, that tells you everything you need to know. Wish him well and end the relationship. As you have been sexually involved, it will be difficult to cut off all ties, but, do it anyway.
If he does agree to this respect and chaste relationship, you need to know if you are even free to marry. In some places you talk to your priest, in others you speak to the Diocese Tribunal (you can find yours online with Google).
If you are not free to marry, then you may ask the Tribunal to review your first marriage to determine if it was valid.
If it was valid, you end the relationship, stop dating and live your life without romantic partner until your valid spouse dies.
Should the first marriage be found invalid (assuming your boyfriend has no prior marriages to be investigated), you would then do some counseling. Your priest or someone designated at your parish can help facilitate these discussions, taking the Church’s compatibility test would be a good first step.
Understand that you need to be 100% good with living with this man, raising children with him, exactly as he is today. That he will not ever change one bit in his politics, religious views, lifestyle choices.