It seems like you have a phobia about commitment and as soon as you start having these doubts you start finding little negative things in the relationship to sabotage it. Commitment phobia is rooted in fear, fear of lost options or fear of making poor decisions, commitment phobia is the fear of having to commit to anything, but especially relationships. On the one hand you want avoid commitments yet at the same time you crave being committed and being with your mate. You may yearn for a deep and intimate relationship leading toward marriage, but fright causes you to second guess your choice in a mate and then you doubt the relationship. This seems to be a repetitive behavior because you have done this with many of your relationship and they ultimately ended with you walking away from the relationship.
In your relationships, you crave what you fears most: love and connection. Some contributing factors can be previous abusive relationships, intimacy issues or traumatic childhood experiences could be causes for this kind of commitment avoidance. Another possibility is that as a child you might have witnessed, or been a victim of, poor role models, or even abusive relationships during those formative years, this can consciously or subconsciously affect the way you feel and take part in relationships as an adult. You may want to find the source of your fear whether it be a past relationship with another man or a past relationship with one of your parents and then take the steps to work through those past issues so not to ruin a good and healthy relationship now. Also a counselor maybe be helpful and helping you find out where this stems from and how you can work through it.
You can possibly get over this commitment phobia by first accepting it and then finding ways to work through it. You can understand that it is completely possible to love someone and still have a mission which drives them to be who they are like, work, school, fulfilling a dream. You must understand and know what you want out of life and know your mission, and staying dedicated to it while being in a relationship. When you start to fear your relationship think back at the fun things you do with him or how he treats you this can help you to keep the positive focus on the relationship while working through your issues with committed relationships. It won't happen overnight so give yourself time, there will be times when you will go back to that fear and it will feel like it's overwhelming you, sometimes breathing exercises are good for relaxing your body and mind. If it gets too overwhelming I would consider counseling for this.