If you don't have a girlfriend yet then you probably just haven't met the right girl. I'm 18 and have never had a boyfriend. I don't feel bad about it. I don't need a dude to validate me, and I'm not going to waste my time on a guy who I don't really care about just to get other people's approval. Same should go for you. You're perfectly fine on your own. Do the shit you like to do, be happy, and eventually the right girl will come along and things will click.
Just don't, whatever you do, be desperate, creepy, or try to hard. Contrary to what romantic movies would like you to believe, stalkers/overly obsessed guys are not attractive.
Unlike some folks have told you, you can have a serious relationship at 18 and being faithful and making it work, some people are able to, some are not. The relationships can get in the middle of your projects or not, depends how you handle the situation. Of course, it requires maturity, but is not the first time I know somebody who got married to the boyfriend or girlfriend they dated in the teen years and are happy now.
But again, you do not "need" a girlfriend. Eventually you will get one, but forcing it only will make you look desperate and scare possible "girlfriends to be". I do not say you are desperate, but it can look like that if you go "in hunting mode".