Check this funny video I have found recently on the internet. You can learn a lot from it. Especially on how not to practice guitar!!!

The biggest mistake guitar students make is not to review their mistakes. If you don’t understand why the mistake happens  you are likely to struggle like this gentleman.  What I would recommend is to break down a song you are learning into really short fragments and work on them separately.  Once you can play each chunk of the song cleanly try to link them together one by one. Be patient and analyze. Also try to figure out what to do to make it better instead of asking why it is not getting any better.

Repetition alone won’t make you a better player in fact repeating mistakes makes you even worse guitarist!!! Practicing guitar slowly is not good enough either. What you have to do is play CORRECTLY and CLEANLY and SLOWLY. increase the speed gradually.