Idk

I've always wanted to be in a relationship, but whenever things start going well, I completely lose interest in the person. I don't know how to fix this. Now, the same thing is happening again, but I don't want to hurt that person by saying I don't want this anymore. However, I know that if I continue with this, both parties will get hurt. I know this makes me a terrible person, not being able to express myself properly and ending up hurting others, but I can't change, nothing I do will make me change.