Depression in relationships

When ever I get deep into a relationship I start to become really insecure about myself and depressed. I start to back tracking through past relationships or people they've slept with (as I've done with my current boyfriend now). All I can think about is him with other women and how much he would have enjoyed the sex etc... he had with them. I literally cannot stop my thoughts and worries and it leads on and on to the point where I get anxious. I go to the toilet up to 8 times a day because it messes with my stomach so much. Has anyone had a similar issue or is going through the same thing? Am I crazy? Or is this normal with some sort of solution ...