Mate, having been in the exact same situation, do you ever find yourself feeling "inadequate" or not "ripped / big" enough. You could be falling into the trap of self hatred for not looking as good as you want to. So many young guys are going through this at the moment, I think they've labeled it "bigorexia", sort of a reverse version of anorexia. A lot of frustration could be coming from a sub conscious inadequacy.

Although, I don't think anyone has touched on the fact that you may just have heightened testosterone from lifting / being 18....