Well there are a couple of things that could be at play here.
First, you are getting older. It's really easy for a teenager to get turned on by everything and want sex all the time. As you get older and more mature, that mellows out. This is perfectly normal and healthy.
Also, drinking (for many people) causes sensation to decrease and pleasure to decrease, and for males makes it hard to get an erection or ejaculate.
Unless your erection problems are getting in the way of actual sex (like you can't maintain an erection long enough to penetrate your partner) then it's not really a problem. I wouldn't worry unless it actually starts interfering with your sex life.
And it could be mental- try to relax!!