For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little:

Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself. Sufficient unto the day is the evil thereof.

COMMENTARY

Precept upon precept; line upon line
It is easy when reading the stories of the saints to wonder how you could ever measure up to their great example. But it isn’t fair to compare yourself at the beginning of your journey to them at their end. Moses was not born as a prophet and lawgiver, he spent nearly a full lifetime growing into that role. Young Moses might not have been ready to stand up to the entire Egyptian nation and rescue all of the Hebrews…but he was ready to stand up to one soldier who was beating one Jew (Exodus 2:11-12).
So long as you are fighting down complacency and actively progressing, then it is alright to not yet be able to do all things. Just take it one line at a time.

Take therefore no thought for the morrow. Sufficient unto the day is the evil thereof.
I think the fear that we might fall away is simply the recognition that we are not yet ready to bear all things. And that is true, we are still unfinished vessels. But God is not going to ask the world of you today.
There is such a thing as preparing for the future, but there is also such a thing as fretting over things unnecessarily. You do not have to succeed today in tomorrow’s trials. If you do not feel ready to face a sacrifice like Abraham’s, that is fine. Simply ask if you are ready to face the sacrifices that are actually before you right now. I have found that I always am.