Is it normal that i'm physically unable to love?

I've been in numerous short relationships and in almost all of them I was the one that ended things. Everything always starts off good but I find my attraction to my partner always vanishes within about 5-7 months, sometimes just 3 months. I've had crushes that lasted for longer though but that was because we never dated. It seems like I only have interest in people when I'm not dating them. I also am reluctant to tell attractive people that hit on me when I'm in a relationship. I don't necessarily lead them on or reciprocate any flirting they do, I just let them flirt with me and am polite. In my mind I'm worried that if I tell them straight up that I'm dating someone then they'll stop speaking to me and potentially find someone else they're interested in. I know going into every relationship that it unfortunately won't last long no matter how much I wish it will, and so I choose to keep people that are interested in me close by. This way when I'm single again they'll be there as something to fall back on and take my mind off the emptiness.

I feel awful doing this because I feel like I'm betraying my partners even though I don't flirt with the other people and have never cheated in my life nor ever will. I have no empathy or guilt when doing the things previously mentioned until a few days later when it finally hits me. I also wish I was able to fall in love and have a long lasting relationship. I feel shitty and disgusting and I'm scared I'm a psychopath or sociopath or something. I wish I was normal