+1 for all Maggior's advice. Clear, defined goals are essential for progress. Another thing I'd aim for in your case is recording your guitar on your computer if you can, or just how you're doing it now, with the goal of recording something that you want to play perfectly. So for instance, make the goal of learning "Back in Black" just the rhythm part, download a backing track of it and record the guitar part when you feel you can play it without mistakes. This way you can listen back to yourself not just practicing, but actually playing something that you believe you are able to play correctly. If listening back, you hear mistakes, then you know what you have to work on. If you don't hear any mistakes, and it sounds really good, then you know you're progressing, and have something to show for it... a little milestone if you will.