Well, school will be better because now you are sober, and hopefully learned coping techniques in rehab, and hopefully you are going to meetings outside of rehab? Like AA or something? If not maybe start doing that and get yourself a sponsor to call if you're feeling tempted. Having some type of support system will help.

Just like PC knowing people who understand something of what you're going through or been through is really helpful!
Also, it is good you are taking only 3 classes. Don't bite off more than you can chew, as the saying goes. Maybe make notes for yourself somewhere if you go manic that pop up or you see everyday saying, "No seriously, don't take on more projects right now. I mean it!" Or something.
Truck... i hear that. No money, broken truck. My car was stolen last year. I couldn't afford a new one. Thankfully my dad gave me his old chevy. But, the breaks need fixing, the heater doesn't work, the cool air doesn't work, the door handle broke off, and it's missing seat belts, so I can't take all my kids at once.... It will be one year Saturday that I have a truck I can't afford to fix.
Why did you do it? Because that's what the suicide demon does to you. It whispers lies in your ears while you're vulnerable and if you're not in a good place to fight back, well bad things happen. That's why its good you're doing better now and sober, that's a good first step to fighting back.
And grandma? Well, maybe for now the best thing to say is "I love you, grandma. Thank you, grandma." It was good of her to bail you out. If all she expects in payback is a little TLC, then that's better than if she's on you 24/7 with "where's my money!"