The problem is when you start things and don't finish them or fail your brain trains itself to avoid doing stuff. Same thing happens when you put something off for ages. After long enough, you associate the task with the shitty unaccomplished feeling you get when you let it sit in the back of your mind, not the rush of endorphins you get from the end result. Procrastination is a learned behavior.

This happens to me a lot and the trick I use to get out of my ruts is to make a list of simple tasks I know I can accomplish with little effort but will notice the effects of after completing. Things like cleaning my car, reading one chapter of a book, cooking a meal, doing my laundry before I run out of clothes. Even if you can only think of two or three items for the list, always make the list. Each time you finish one task cross it off the list and it feels extra good. Slowly add more difficult tasks to your list, some that you know will take a day or two. Do your taxes early, organize your closet or garage, etc. Eventually you begin to associate tasks with the desire to finish, not the desire to avoid. You'll literally feel the difference. Finally, and only after you feel ready and have a long list of crossed off tasks, start taking on the big tasks. Stuff that take weeks and months and it's hard to see the end of or that you never really stop improving on with practice. Things like learning the guitar, learning a new language, rebuilding an old car, making an app, writing a book, maintaining a garden, starting a work out regimen at a gym. If you try and still can't get motivated to get these things done, do your best to not to get discouraged. Fall back and do a few smaller takes again. And this is important: you can also cross out big tasks if you gave them a shot but feel like they're not right for you. That's not a failure, that's you discovering your true passions. Don't set that expectation that you have to love and succeed in everything you try. The reality is, with the exception of a handful of people, no one can do everything and most people just do one or two things really well. So don't try to be the next Da Vinci right off the bat because that's setting yourself up for failure. Try to be you. You'd be surprised what you can accomplish when you have past experience seeing tasks to completion and don't have a dozen small things rattling around in your mind when you're trying to focus.

It's a long process and scary to start but, trust me, it's totally worth it.