Well, talking to a professional therapist or counselor can sometimes help with any kind of psychological trouble. I know from experience that it's hard to talk about personal issues with anybody like friends or family who's emotionally invested in your personal life, since the last thing you want is for those concerns to cast a dark cloud over all of your interactions. But with a professional therapist, there's not that concern, and chances are they have experience with other people who have faced the same issues. If you're still in school, a school counselor might be able to help you, or at least direct you towards other resources. Talking with a school counselor is usually free, and it's often very easy for you to book future appointments after trying out some of the strategies they recommend.

I think you rock.

I'm praying for you as I write this.

I don't want to pull the "I know how you feel" card, but I'm at least quite familiar with the self-esteem issues.
 
My best blanket-advice for any situation? Goals. You would not believe how much this helped me to turn my life around. You really have to just sit down and ask yourself, "What do I want my life to be like in 1/5/10 years?" Figure out what your goals are and then write them down. You should also write down what needs to be done to achieve these goals with no lousy excuses for why they can't be achieved (e.g. I'm not smart enough, it'll be really hard, I'll probably fail). Then the only thing you need to do is follow through. Once you make it your entire life mission to achieve these goals, things start to turn around.

Takuma Nuva

Gonna have to agree with Tak a bit here. It can be difficult, but try to start looking at yourself as someone with potential, I guess? Just starting to think you're someone who can accomplish specific goals can be helpful. It may not happen all at once, but little by little it's possible to start to look at what you are vs what you want and start thinking of how that can be accomplished. It's not much, but that way of thinking can be really beneficial in my opinion.

(Also Aanchir's advice is good too. Having someone to talk to is always a good thing)

Thank you so much for the advice, everyone. I'm speechless, this is amazing.

@aanchir: I agree with it being difficult to talk about these things with friends and family, but I'm not sure I'm completely comfortable talking with a school counselor; it's certainly worth a try, though. A professional therapist might be good as well.

@Obsessionist: thank you so much <3

@Takuma Nuva and Tekulo: Setting goals seems like a good way to get things on track. To be honest keeping with something isn't my strong suit, but I'm gonna follow your guys' advice and try to work towards something(s).

Thank you all again. This totally rocked my world.