I've also struggled with everything you're going through. If I didn't feel like I was giving 110%, then I was failing. It was completely black or white, and when you place such high expectations on yourself, you will always come up short.

One thing that helps me is to think about how much I need to do to be satisfied. For instance, if I have an exam, what can I reasonably expect from myself to feel prepared? Figure it out and then ask yourself, is this reasonable? Heck, ask someone else if they think it's reasonable. If it is, then you need to plan when you'll do what. Time management is key. But at the end of the day, even if you didn't get everything done, you have to be gentle with yourself. The world will not end, I promise, no matter how much it feels like it is going to. Be flexible and be gentle.

Another thing, which someone else already mentioned, is don't think about the big picture. You'll worry about that when you get there. Break everything down into small, manageable, reasonable goals. When you start to worry about things in the future, just remind yourself, you'll deal with that when it happens. Just focus on what's at hand. If there's something you can do right now to work on whatever is worrying you for the future, then do it. If not? Don't worry about it.

It's going to take a lot of retraining your brain. It's hard, and I'm still working on it myself. But you have to keep countering your negative thoughts with positive ones. Sometimes it helps to write things out. For every negative (ie, "I'm going to fail"), write out a positive ("I'm going to do the best I can and that's all anyone can ask of me. I'm not going to fail"). Even if you don't believe it, keep doing it. As my T used to say, fake it 'til you make it.

Good luck! Remember, you're not alone! You can get through this!