A common problem many people encounter with Parkour is frustration with their level of progression. The usual trigger for the onset of these feelings is training with people who are of a more advanced skill set, or training with friends who are progressing faster than you are. This frustration, if not dealt with, usually leads to the individual giving up on their training. This is unfortunate and unnecessary. It requires a change in perspective and an application of the spirit of Parkour to overcome.
Time should be spent reminding yourself of the purpose of Parkour and the spirit in which Parkour is practised. A traceur overcomes obstacles, when something is too hard they do not give up. They train and think about ways to get past whatever is in their way, it is the nature of the discipline. Just because the nature of this specific obstacle in not physical (feelings of frustration, not the fact that you may be physically weaker) does not mean it cannot and should not be approached any differently. As a traceur you should be training for yourself, not to impress others, nor to compete with others.
We are all different. We all come with our individual strengths and weaknesses. Some people get strong quickly, others do not. Others pick up skills quickly, seemingly with little effort, while some have to train exhaustively to pick up the same skills. If you are to get over feelings of frustration you must come to grips with the fact that we all progress at different rates, and that IT IS NOT A RACE. Maintain focus on your own efforts. Do not compare yourself with what your training partner of friend is doing today. Focus on what you could do yesterday and what you can do today. Seek to challenge yourself with the progress YOU make. Because if you are consistent and active in improving your own ability then you surely will. But you have to be realistic and persistent. It is not something that happens overnight. It is something that is a work in progress and can take quite a time to adjust to. But if you can change your perspective, remain persistent and continue to train regardless, then you will find the pressure you put on yourself to be less of a problem and your training to be more enjoyable than it would be if you keep comparing yourself to other people.