You have an overwhelming fear of abandonment that you need to work on independently of your girlfriend's other relationships. I would like to point out that what happened when you thought she was breaking up with you was that you were paralyzed by fear and you became 'traumatically bonded' to her. (I've been there, too).

This type of bonding is unhealthy and makes you codependent of her so you lose control over your own boundaries, decision making process and life. Not being an independent partner will also mess incredibly with your relationship(s).

You need to start working with your abandonment fears from the perspective that your brain is tricking you into believing that being abandoned by your attachment person (aka girlfriend) is a question of life or death - like it used to be when you were an infant/toddler/kid. Your brain is wrong. You are a grown-up fully capable of taking care of yourself and even be happy if she left you. Basically, you need to visualize yourself being abandoned by her in 1001 ways - and every time you have to imagine an alternative solution to that situation that makes you relax and be ok with it. Don't stop until you've got a hang on this. It's the only way you can become an independent partner and make healthy decicions for yourself.