Why are you trying to have a successful relationship with someone new (even though she's also an ex) when you haven't yet processed your last relationship to the point of being at least somewhat indifferent to her?
You are using your new girlfriend like a human bandaid to help you get past your pain. That is quite selfish actually and codependent in its pathology.
I do hope that your therapist is proficient in dealing with clients that are codependent so that you and he can work on you getting past some bad habits you've developed.
In the meantime. Put an elastic band around your wrist and ping it hard when you find yourself wallowing in your memories of her. You need to immediately change the subject of her when she pops into your head and the rubber band will make you consciously aware of your wallowing. (It's called aversion therapy)
I do hope that you are doing zero contact with this girl. You can't quit smoking if you keep having a drag off of a cigarette. Same thing with this girl. You won't be able to quit her either if you keep taking little hits of her attention.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion