Sometimes, when we do a particularly powerful mitzwah (even if it seems like a plain simple mitzwah to us) two things happen: (1) HaShem makes it a little bit harder for us to perform the mitzwah, which makes the mitzwah that much sweeter when we do it anyway; and (2) the Yetzer Hara (the evil urge) makes the performance of the mitzwah extra bitter in the hope that we will regret ever doing the mitzwah. If we regret performing a mitzwah (has v'shalom) it might be taken as if we never did it at all -- just as voicing our regret for an aveirah (a sin) is the essence of teshuvah and erases the deed itself.
Armed with this knowledge, it is important to note than when we set out to do a mitzwah and encounter an obstacle, we must gather additional determination and press onward. And when we feel remorse about the cost (emotional, physical and even monetary) of the mitzwah, it's time to thank HaShem for giving us such an obviously precious mitzwah. If it weren't so precious, the Yetzer Hara wouldn't be trying to undo the mitzwah even after its completion.