Dealing with severe jealousy issues

I never knew I could become this way and for the longest time I was always in denial. But now that I've come to terms with the fact that this is a serious mental problem for me, I'm wondering if I could get some advice from you ladies.

It seems almost everyday I am worried about my man when he's away at work. I go through so many scenarios in my head, that even though I KNOW he's better than that, I convince myself that I'm being naive and he IS catching feelings or flirting or being sneaky.

My thoughts have led me to have nightmares throughout the months of me catching him cheating and beating the absolute shit out of the woman.

Sadly, this toxic jealousy has been a problem for me for almost every single relationship I'm in and I'm so surprised my SO hasn't left me...

Please help me. Has anyone dealt with this before? I can't afford counseling right now otherwise I'd be there instead of on here...