Your relationships sound scripted. Like you know the beginning, middle and end before you turn the first page of a relationship. And, because you are sure of the outcome, you avoid getting attached, because you know that you will end up being hurt and rejected, again.
Whenever you find yourself playing out a script, or caught in a repetitive scenario, there is only one reason why this is happening. The mind is replaying some core scene of childhood that hasn't been healed.
So, you need to find out what childhood trauma is being replayed for you. It sounds like you are reliving an abandonment theme. Someone made you trust him (or her) then rejected you. By now you have probably realized what part of your past is being replayed.
Now you need to figure out what healing your mind needs to accomplish. Sometimes the replaying of a past wound is the mind's way of exorcising the bad feelings. By reliving the painful feelings of the past, feelings weaken until they are fully worked through.
Sometimes the mind wishes to replay the past in order to achieve the happy ending that I talk so often about. Is your happy ending that your boyfriend will stick around and not reject or abandon you?
In order to obtain this happy ending, I suggest that you enter therapy. In good therapy, you will have the opportunity to replay the abandonment feelings and obtain the happy ending (the therapist will stick around and not leave you).
The ideal for you is to stop replaying the old abandonment theme in real life, where the stakes are higher and each new abandonment adds insult to injury.
Also, be aware of the people that you choose to date. Ask yourself: Is this person an abandoner? (You seem to sense from the beginning that the people you are choosing are abandoners.) If your answer is yes, then avoid these people. Also, ask yourself, do I have a chance of receiving a happy ending with this man? If your answer is no, then run for the hills.
If necessary, have no relationships until you work this issue out in therapy. Being alone is better than being abandoned again and again.
Lots of luck. You sound lovely and worthy of a love that sticks around.
Let me know how you make out.