"The world we have made, as a result of the level of thinking we have done thus far, creates problems we cannot solve at the same level of thinking at which we created them." - Albert Einstien
Far too often we are told as children not to be boastful, not to get our hopes up. Unfortunately, this often leads us to have low expectations of ourselves. I've seen it far too often in my myself and in even my best students. We don't believe we can do certain things. We can even feel good talking about our problems.
One may practice every published study, every exercise, practice 10 hours a day, go to hundreds of teachers, and yet not improve very much because of a negative mental ruling state. If you don't expect to get better, you won't. You have to expect it. You have to see yourself playing well in your mind, believe it to be possible, and expect that it will come.
This is not wishful thinking. This gets the subconscious mind into a position where it starts to look for answers. Expect to find the answers you seek and they will begin to come. Too often we don't expect the good we desire. We focus on the present reality as a fixed Truth. Our present is only the effect of our past thoughts and feelings.
Begin to watch any thought or statement which describes yourself or your relationship to the guitar. When you think or say "I can't do ________" or "I'm not talented" or "I can't memorize" or "The guitar is difficult" etc. - realize that you are programming your mind. It doesn't matter how many years you've had these issues - start changing the way you think and things will start to change. There's no reason we can't do what we want other than the belief that we can't. Whether you believe something is true or not - you are right.
Realize that what you expect and see clearly in your mind will come to you with time. Your visions, when backed by unwavering faith are just like physical seeds. They will grow if they are taken care of and protected. Don't let another person (or yourself) tell you you're wrong because you've never been able to do it before. Don't let the past decide your future. Use the will to stay focused on your vision. Napoleon once said "I see only the objective, the obstacle must give way".