Gut feeling vs insecurities

Things feel off but I'm not sure if its a gut feeling something is wrong in the relationship or if its my insecurities. Weve been together for years now. Ive never found proof of infidelity. He comes off flirty because he is noce to everyone and outgoing and other women take advantage of that- several even after they found out he was in a relationship.

I've always had trust issues and he knows this. I guess what I'm wondering is how do I know if I'm not trusting him or if I'm not letting myself trust him? I want to believe he's a good guy but I grew up in a world where men were abusers and cheaters.