If you do everything I say here, it probably won't make you a dynamo overnight. It will help you avoid backsliding, though. Over the next few years, if you are diligent, you will learn new habits.
Step 1: Throw your TV and any video game systems you have in the trash (or more sensibly, disconnect them and put them on craigslist so they are out of your house today). The whole point of startup culture is to give up some fun now (wait, working at a startup IS fun) in exchange for a huge pile of money later. You want to work 80+ hours a week, and probably have a girlfriend too. When does playing Xbox fit in there? In a few years, when you are the founder of a hot, funded startup, you can have an XBOX in your game room. Or you can play it now, and maybe the VC fairy will be impressed by your GTA skills. -- The point is, every minute spent fucking around is a minute you could be working or learning new skills.
Step N: The next time you are wondering "why can't I be productive", imagine a little bell ringing in your head. The bell is there to remind you to sit down and work. If you find yourself browsing reddit or hn or some other site, add entries to your hosts file so that you CAN'T browse those sites. Often the anxiety that keeps a person from being productive is at it's worst at exactly those times that you would otherwise be the most productive. Never visit any distracting site like that from your work area or from your work computer.
Step Na: Give yourself a little goal to accomplish (write it down) when you sit down to work, and don't let yourself get up to do something else (besides restroom breaks) until you accomplish the little goal you wrote down. When you do accomplish it, imagine another 'you' there, and see if that other 'you' would accept the quality of the work if you were an employee. Once you give the work a thumbs up, force yourself to take a 15 minute break. Go outside for a walk, call a friend, just get off the computer. Once you are comfortable with this micro-routine (say after 3 weeks of doing it consistently), check out GTD. Resist the urge to go full on GTD now, however. Most likely you will get distracted setting up notebooks and org-mode or other yak-shaving instead of getting actual work done.
Step X: Make an appointment with a psychiatrist that has experience treating adults with depression, ADHD and anxiety issues. It may help a lot, and it can't hurt.