I know I've been where you are before. Being depressed isn't fun, and it's an uphill battle to overcome it on your own sometimes. Not having a car during this time is really tough...I understand - I had no car until I was 18. It must be even worse to have one (and all the freedom that comes with it) and then lose it, like you did. Understand that you do have the means to put your car situation to rest, the only thing you need to do is get the ball rolling. You know what you need to do, just go ahead and do it.
Your situation at school is very important and believe it or not has an effect on the rest of your life. You're probably in your junior or senior year, which are usually at many colleges reference when reviewing applicants. Now's the time to pull up your bootstraps and give them the best of what BV has to offer. Your seeming inability to pay attention could very well be tied to the Diet Pepsi. The excessive caffeine is overstimulating your body and mind. Your body doesn't want to sit still, and your brain can't focus on any one thing. What's worse, your depression *could* just be the result of coming down from a caffeine high. After running at Mach 5 under the influence of caffeine for so long, your body can get tired and "crash". I know the feeling from being on diet pills that had caffeine in them (unbeknownst to me). Depression is as much a physical condition as it is a mental state. If you do dump the Diet Pepsi, don't go cold turkey. Drink less and less over time. When getting rid of a "bad" habit it is always helpful to replace it with a "good" habit. Instead of drinking Diet Pepsi, drink some juice or water. Instead of chocolate, have some fruit (raisins, blueberries, and strawberries are great for fruit-newbies, and clementines are in season...yummy). This might sound weird to you, but put it this way: should you decide to go to a doctor, your best bet is to have a body that's working as it was designed to, without the help of drugs like caffeine.
If I may throw some advice to you that you may or may not want to listen to, why not scour your local churches for youth groups? Based on the one run out of my church, it's not what you're probably thinking. They're usually full of teenagers much like yourself (or much worse) who hang out every so often to play sports, arcade games, go on camping trips, help each other with homework....you name it. Plus, there's usually a youth pastor there that shares the Bible with you in a way that'll make the words jump off the page to you. In most cases you don't have to be an active church member to join.
I realize this is a bit long and preachy, but I hope at least one sentence helps you in some way.