The first step to solving a problem is recognizing it. I applaud you for facing something that must be very difficult for you. I am very glad that you haven’t complicated things by getting addicted to drugs or alcohol.
It may be that something happened to you either while growing up or when you started having romantic relationships that made you pessimistic about whether someone can really love you. You very much want a relationship, but you fear that you will be rejected. One way to resolve that dilemma is to ruin a relationship before the rejection you imagine actually happens. That makes you feel more in control and preserves your self-esteem. You “fire” your girlfriend before she can “quit” on you (as you are sure she will do).
You are wrong that you cannot stop this. You are only in your 20s. You are not hopeless. Your letter shows that you are smart and insightful. Now you just need some help to overcome your pessimism. If you could do it on your own, you would have done it already.
Unfortunately, I’m not familiar with the services available in Vietnam. If there are counselors, I suggest you see one. If not, you might want to join one of the support groups here on PsychCentral.
You have a long life ahead of you. It’s wonderful that you want to solve this problem now so you can enjoy an intimate relationship with someone who loves you.
I wish you well.