Pattern of Strong Aversion

I noticed that when I engage in a task I am very able until I reach my limit, namely the point where I had enough, where effort and suffering kicks in. I noticed: - When I reach that point I lose energy, thinking becomes more difficult, confused. With something I enjoy it is the opposite. - When I reach that points demotivating thoughts as "This is worthless," "What's the point of doing this?" occur. Even in meditation I am able to go a long time without effort or difficulty if I feel like it, but when there is resistance I fall asleep or want to do something else. My question is: how can one increase their tolerance to physical pain and effort, suffering, etc. -- all the things linked with aversion to negative states? (Keep in mind that when I meditate at the wrong time I fall asleep or encounter difficulty). Thank you.