I'm in a relationship but I keep breaking down when my girlfriends ex's are mentioned and we have talked about it and that but it doesn't help it still happen I'm worried I'm gunna end up screwing the relationship up
You're right. You will screw up the relationship.
What are you worried about? She's not with him any more she's with you. Stop comparing yourself or feeling insecure. You can not change the past.
If it continues see a counselor/therapist.