I once heard this powerful idea from R’ Mattisyahu Salamon Shlita.
Most Rishonim say that the last Nisayon of Avraham Avinu was the Akeida. However, Rabeinu Yona says that the last Nisayon was the burial of Sara.
The question is obvious. After having been through the Akeida, what was the big deal about having to haggle over the plot to bury Sara?
The answer is that sometimes a person feels after having been through a very hard Nisayon "HaShem, I have done my bit, leave me alone already!" and Avraham Avinu had every reason to feel this way. He had just been through nine difficult Nisyanos. Surely, he had proven his loyalty to HaShem! But in this world, there is no let-up, and that was the final, most difficult test.
So for all those who have reached 90 days (like me) - the battle has just begun!