I understand that Jesus has experienced the same weaknesses and temptations, but what about uncertainty? Or even sin itself? How can He understand what it is like to not know anything for certain when he has known everything from beginning to end. God has never sinned, so how can he truly understand us without experiencing these two things?
Tackling the sin part
Highlighting a false premise, that one must experience something for one to know/understand it. It is worth noting that experience is a type of knowledge ...
This is actually quite a loaded question philosophically, theologically, etc. Theologically, there is always mystery.
13 if we are faithless, he remains faithful—
for he cannot deny himself.
Beginning here we see the reason. The mistake is to view sin as something, while it is actually an absence of something. cf. Evil | New Advent.
Jesus, is God, therefore, ALL Goodness, ALL Truth, ALL Love, etc. Hence St. Paul, for he cannot deny himself.
Not only did Jesus not sin, he could not sin (he was impeccable cf. The Incarnation | New Advent).
Herein is the mystery: Jesus, as Son, moves naturally and necessarily toward the Father, yet also freely.
Sin understood here as doing one's will in opposition to God's.
[The] Uncertainty part
The Church teaches that Jesus had a three-fold knowledge: acquired, infused, and beatific. Delving into this would answer if Jesus was ever uncertain; too broad a topic to be incorporated in this answer.