I heard that you can retroactively influence the Mitzvot you already did. For example, let's say I put on Tefilin everyday, but, in one specific day, I felt really sick and I have to make a really big effort to also put on Tefilin that day. If I do manage to do it that day, it shows that I was ready to make this kind of effort and I would do the Mitzvá even in difficult situations.
I heard that the merit I had putting on Tefilin on a normal day will be increased after this situation, because H'shem will recognize that I would be willing to do the Mitzvá even if it was hard for me and involved effort. This way I proved that I don't put the Tefilin just because it is easy, but because I take it really seriously. Being so, I deserve, retroactively, to earn more merit for all those regular Tefilin days.
It this correct? If so, could anyone tell me the source?