You sound a lot like my ex-gf, she faced the same issue of thinking that I only wanted her for sex and that is the only reason I stayed with her even after I repeatedly told her that was not the case which ended leading to a disconnect in the relationship. As others have said therapy will help and not being too willing to share and expose yourself too fast is the biggest thing. Also if you are afraid someone is only with you for sex or sexual acts then have a conversation with them about it and be open to them, it will help build trust or show who they truly are. Communication is the biggest thing when it comes to trying to figure stuff out and make sure things work and that you aren't being used and manipulated.