What is the hardest mitzvah to do on or for Pesach? Is it the cleaning? Stuffing multiple k'zeisim of matzah down your throat? Maror? I think the real answer is none of the above.
The Shem m'Shmuel asks why is it that the Torah devotes a seperate command to Bnei Yisrael to take the korban Pesach on the 10th of the month -- we usually do not find in the Torah that the preparation for a mitzvah is itself a command in addition to the mitzvah itself. He answers by quoting his father, the Avnei Nezer, that although seyagim are normally only derabbanan, there is one exception to the rule -- "md'var sheker tirchak". Here the Torah specifically warns against even coming close to sheker in any form whatsoever. The Shem m'Shmuel explains that taking the korban Pesach involved forsaking any committment to avodah zarah, the ultimate form of sheker. "Mishchu u'kechu -- mishchu y'deychem min avodah zarah." Therefore, not only is the act of the korban pesach itself a mitzvah, but every detail of preparation and everything associated with that act is also counted as a mitzvah just as the harchakos and seyagim of avoiding sheker are all counted as mitzvos d'oraysa.
The Divrei Chaim explains that Hashem "introduces" himself in the Aseres haDibros as G-d who took us out of Egypt as opposed to the creator of the world to emphasize that although we were not deserving of redemption, Hashem nonetheless did redeem us; though we may be unworthy of having a relationship with Hashem, "Anochi Hashem Elokecha", G-d is with each of us no matter on what level we may be at.
Let's be realistic and honest -- do any of us really believe in these few remaining days until Pesach that G-d is sitting and ready at a moment's notice to extricate us from galus? I mean, look at the world, look at our own 4 amos -- are we the people who you think deserve geulah? Al yish'u b'divrei sheker!
Now you know the hardest mitzvah of Pesach. To really believe that no matter how bad things look we can all be sitting in Yerushalayim together in a few days is even harder than swallowing a chunk of maror. And to deny that this is true not only confuses the real emes with sheker, but is a form of avodah zarah. If G-d is the only power in the universe, can it be beyond his ability to rescue his children no matter how far they have fallen?
The Ropshitzer taught "t'chila mikraei kodesh" -- the first step of becoming holy is "zecher l'yetziyas Mitzrayim" -- remembering that a yetziyas mitzrayim that can rescue a person from the 49th abyss of tumah is but a moment away.