Using the will in our work is extremely important and is often overlooked. We want to use the will to keep focusing on what we want - not on what we don't want.
In the case of practicing the guitar, we want to decide on what we want (better tone, more accuracy, more colors, running through the piece without stopping, etc.) and then use the will to stay focused on that - regardless of the current circumstances. Realize that staying focused on what you want will bring it to you, but it might not happen immediately - although sometimes it does. Often I have to simply tell a student to focus on getting a better tone, and it comes. Or I ask them to focus on being more accurate, and it comes. I don't have to explain anything or introduce new techniques. The student simply has to make a decision of what to focus on and then use the will to stay there. Then the mind / body will start to find solutions.
If there are no decisions made in our work, we can become easily distracted and overwhelmed. Our job is to focus on what we want to improve and use the will to stay focused on that during the session. 15 minutes of this kind of work is worth months of unfocused practice. Remind yourself by saying "I want to play my piece at this tempo" or "I want to memorize this passage" or "I want to get a better tone". Write down the idea and post it all over your practice room. Keep reminding yourself of what you want to improve. It takes time to build the momentum for new ways of playing - just stay on it.