Why is it so hard for me to get close to someone?

I've noticed it's extremely hard for me to get close to people, I have trust issues and my best friend told me I have a wall put up. The thing is I never feel good enough for someone, as dumb as that probably sounds, I always feel like someone could do better than me, I'm not just talking about my looks it's everything about me. I don't know why I'm like this maybe it has to do with my childhood. My childhood wasn't the best, my parents were druggies, I was adopted by my grandparents. My parents stole from me for drug money when I was younger, and I rarely remember seeing them sober they were always on something when I saw them. Well anyways my Dad died a few years ago from and the last time I talked to my mom was on xmas, she never did keep in touch I was always the one contacting her, but she is off drugs now (I think) so that's good. I'm 19 years old almost 20 and I don't think I should be feeling this way, I mean I don't think it's healthy. I don't feel like I'm relationship material for anyone, and I never get my hopes up, in fact I usually think "it will never happen let it go" and I let it go. My friends have tried to hook me up with guys and I always refuse and they always ask me why and I say they're not my type, when deep down I just don't feel good enough for them and I would honestly feel bad if they were seen with me or I would feel like they were doing it out of pity. Will this feeling pass? I've felt like this for a VERY long time, should I seek professional help? :/ Thanks.