HELP!

So every few weeks around my period I turn into a really paranoid mess. I convince myself my bf is cheating on me and I’m going to get cheated on, I have dreams he’s cheating on me where I’m constantly thinking about it all day.

When I’m in a good headspace I know he isn’t but it gets so bad when I’m not that it’s really ruining my life for that few days it’s happening even up to 2 weeks sometimes

Please help