First, you should see a therapist, you don't need anyone's permission to do this at age 26 and I think it'll help to have someone to bounce ideas off of in person. Find one you click with and feel like you can open up to.

Second, you say you like being negative, I understand how that can feel. BUT as a former pessimist, I know that I like being positive even better. It's kind of refreshing to be able to see the positive in every situation - and trust me, there IS a positive in almost every situation (sometimes it's a lesson).

Third, replace any negative hobbies you might have (like playing too many video games, using drugs, overeating etc.) with other positive hobbies like cooking, writing, or starting a project. Exercising is probably the best thing you can pick up. You'll find that exercising will increase your serotonin levels in your brain (making you happier), help you FOCUS (good for your studying), and even help you sleep. Plus, as your body changes, you'll get more and more confident.

Fourth, try finding an online community (like the one on PlaceboEffect.com) that offers support from professionals AND people like you. That site also gives you life purpose advice, so that might be a good place to start.

Fifth, keep asking for help. Posting on sites like this will give you insight from other people who have come from all walks of life.

I hope these tips help you! They've helped me when I've felt the way you do!