Strange as it seems, a person's basic personality affects how they learn music and how they play. That's why we give our students personality tests. In the Myers-Briggs temperment sorter, Sensory Judgers (SJ) are about 2/5 of the population. About 1/3 of our students are this personality. A characteristic of this type in playing music is that they are good with noticing and mastering details in their songs. A weakness, though, is that they have trouble holding long notes and long rests. Isn't that weird?!? But it's true. So the problem is that as this type of musician hits a long note or rest, they tend to cut it short and skip beats - it's as if they can't wait around for that darn note to complete; let's hurry up and get on with it. If you have this problem, you're probably an SJ. AND, to fix it you just need to start noticing these long notes and rests and stick in your head to hold them their full length. And ALL personality types have various strengths and weaknesses.
Instrumentalists: keep your fingers directly over the frets or the keys - your fingers naturally bunch together, but for speed, spread 'em.