"Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy." - Thich Nhat Hanh
The underlying feeling when practicing should be one of play - experimenting, being creative, being spontaneous, enjoyment of the present. This is how we learn things best and easiest. When the mind is playing and enjoying the process without judgment, it becomes inspired - we get wonderful ideas, feelings, intuitions. This guides us towards right action.
Even when a technique is particularly challenging, we can still retain the attitude of play. I remember in the Fall, sitting in a park and watching a few skateboarders honing their craft. One boy was trying to do a trick where he would lift the skateboard while he jumped onto a bench, and then land back down onto the ground cleanly. He tried and tried, but wasn't getting it. However, he wasn't getting upset - he was playing with the technique. Through playing and experimenting, his mind/body were open to solutions. When he would fail, he just learned a little from it, moved on, and went on to the next try. His mind was not on judgment - which drains us of energy and inspiration. He was just feeling the sensations for each jump with a sort of intense curiosity. Of course he wanted it to be right each time, but he didn't mind that it wasn't. Failing was just a way to understand his instrument better (body and board)- it wasn't something to get upset about or to take personally. He did nail a few jumps towards the end, and although he didn't become fluid at the technique, it was obvious to me that he was going to get it with this kind of practice.
Play doesn't require perfection in every moment. Do we enjoy games only when we win? Of course not. Why should practice be any different? Think of it as a sort of game. Watch yourself and be careful not to fall into harsh criticism of your self or your work. Be interested in the failures as well as the successes - learn from both and stay open to the ideas that come from both. Some of your best ideas will come from failing - if you are open to learning from it.
The attitude of play is one of the key factors in mastering any endeavor. If you read about anyone who is outstanding in their field, you find over and over again that they enjoy their work immensley. They don't do it for the money or fame. Their work is a sort of play for them. Enjoy each step and you'll see that things get easier - as you become easier on yourself. If you have an attitude of play while you work, in many ways you have already succeeded.