18 Year old feeling a little depressed

Hello, I’m new to these forums. Sorry this is my first post on these forums, but I’ve been looking for advice and I’m not comfortable with talking to people I know in person about this stuff yet.
I have this constant feeling, a feeling of doubt or of being unsure or something. When I question my faith in God and in Christ, I tell myself that I’m obviously a Christian. I even look back to a moment in my life when I feel like that was the moment I was saved, as it was a moment when I first truly understood the meaning of the New Testament. I’ve told myself that I have received the Holy Spirit. Yet for some reason that I don’t understand I feel uneasy about my life in general.
I just graduated from high school and I feel uneasy about moving on into the real world, so maybe that plays in to this a little bit. I’m also pretty anti-social and I only have 2 or 3 friends and I don’t really see them outside of school, which is now over. I get along fine with my family, but it’s not like we’re really close or anything. We all kind of do our own thing for the most part, but I know we love each other. Still, I don’t know why I feel uneasy all the time. I tell myself I’m saved, but I’m questioning it all the time, for pretty much no reason at all. This uncertainty is completely unreasonable and it makes me really angry that it’s there at all. I wonder why I have this problem assuming I’m truly saved. I have no hostile feelings towards God, but I still wonder why I can’t be happy.
I used to have some anger issues in the past, and I also used to be pretty open with people as well. Over the past year or so I’ve become ashamed of how I used to act and I’ve tried to become more stoic in various ways. I think maybe I have overdone this and have identified with that personality too much. I don’t feel much of anything anymore besides a persisting frustration. I can act happy in front of my friends, and I sometimes do feel happy. But when I’m by myself the only thing I do is annoy myself with my constant thinking. It’s like I’m driving myself nuts with my various thoughts and this stupid persisting uneasiness.
I pray every night and I started reading a chapter or more of the Bible every night to look for answers. I think I’m a decently smart guy who usually has a handle on things, but I’m frustrated that I don’t seem to be getting anywhere. I wonder if the devil is just screwing with me or something, but then I wonder why this can happen to me if I work so hard to get closer to God.
General advice would be appreciated. If you happen to know of any useful Bible verses I’d like to read them too. Hopefully something can apply, but I don’t know much about the Bible yet so I don’t really know where to look for help.

Thank you both for your thoughts.
Shawn, I think I do tend to beat my self up on occassions. I sometimes feel like everything is my responsibility. I mentioned that I'm a decently smart guy, which is why I feel like I should be conscious enough to understand my problem well enough to solve it. I feel like I should be able to get through my frustrating times by thinking things through, but it only gets me agitated. I want to be able to give God some of the burden to carry, but I feel like it would be wrong for me to ignore my issues.
Criada, I have heard that depression can happen for no reason. In fact, someone from my school that I know a little bit was hospitalized for depression once, and I asked him what he was upset about and he told me nothing (as in he spoke the word 'nothing'). I thought it was strange that he didn't know what was wrong, but now I guess I know how he feels a little bit. I doubt my problem is as serious as his though. I really liked Psalm 13 and I was surprised at how much that sounds like what I'm going through. I will pray this and tell myself to keep praying no matter how I feel. That I need to trust God always
I feel like I behave like a saved Christian would, but I still don't understand why I question myself all the time. I wonder if I just have no confidence or if I'm just thinking about this too much. Or maybe it's not really my fault and it really is the devil messing with my mind somehow. I also think that maybe being more sociable would help. I also find myself bored quite frequently, so maybe I need to find some new hobbies, lol.