Before you begin a self-training session RELAX, this ain't Wall Street. You can't lose riding a mountain bike. If you are working on a technique and you fail two or three times in a row, STOP!! Do something else and try again later. This is called "Training To Failure" (positive progressive training; pushing the envelope). If you push a training session beyond three successive failures you are "Training To Fail" (negative regressive training; more pain than fun). As you become more adept at self-teaching and pushing yourself appropriately you'll be able to discern where good (beneficial) training ends and bad (regressive) traning begins. [Hint: lack of fun marks the spot.]
This excerpt from a book on learning how to mountain bike makes a lot of sense about how to practice anything, music included. For one thing, it makes clear that when you're learning to "self-teach", you need to stay aware your overall mental/emotional state as well as the details of technique. How you go about learning makes a huge difference in outcomes. Without mindfulness, practice for practice's sake can be harmful.
That first sentence is also a great way of looking at motivation. If you're trying to make a living or get a scholarship with your music making, how you approach practicing will be different from someone doing it just for fun and personal enrichment.
I found this over on Kevin Kelly's Cool Tools site. He was one of the people behind the Whole Earth Catalogs and this site is an internet version of that wonderful idea.