As stated above, trust is earned. But if you find yourself thinking this way with EVERYONE (like with family, etc.), then you may want to see someone to talk about it. However, if it is just your romantic relationships, you may be just a little scared yourself about committing and really letting yourself feel totally. The more you open yourself up, the more risk you have of getting hurt. Perhaps you are just scared of getting hurt, so you put up a wall not allowing yourself to trust anyone.
My advice, just relax a little bit and let things happen as they will.
Good luck!
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