A good counselor could help solve this problem. You did not detail what you meant by self-sabotage. Do you stop calling the girl? Are you shy? Do you abruptly end the relationship? Are you unable to begin a relationship? How much are you able to interact with women? Are you able to approach them? Do you begin a relationship but want out when it starts to get serious? These would be important details to know.
Therapy could help you learn better communication skills or allow you to delve deeper into the issue of why you withdraw. It also could help you explore what led you to retreat from the opposite sex as a teenager. Was it precipitated by an event? A therapist can help you realize that you are no longer ‘that’ shy teenage boy. He or she can also teach you how to behave in adult relationships. At this point I suspect that much of your fear has to do with not knowing how to interact with a woman. If you’ve basically isolated yourself from women for the last 15-20 years then it’s no wonder you have fear associated with engaging in a relationship. With a competent therapist these problems are correctable.
You could also try reading a number of self help books on the subject of relationships or self-esteem. You could try group counseling or assertiveness or interpersonal skills training workshops. These may also be helpful.
If you are not interested in therapy you could try self analysis. You could keep a daily log of your thoughts related to relationships or women. For instance, if you see a woman walking down the street and you want to approach her but don’t out of fear, write about this experience in your log. This might help you see a pattern in your thinking over time. If you see a pattern of distorted thinking then you can then work to replace your thoughts with more appropriate or logical ones. If you attempt self help or analysis and are not successful then you might want to consider counseling. Thanks for writing.