Jesus may have been sad, scared, and worried in the garden, but he didn't despair. Someone who is in despair doesn't say, "... not my will, but yours." That line is a sign of total trust in God to carry things through according to the Divine Plan. Despair, on the other hand, implies a belief that God is not good/powerful enough to bring goodness out of a situation--that there is no hope for salvation. And despair in general--the mortal sin--is different from despairing of a particular thing. E.g., at a certain point, Jesus may have despaired of avoiding crucifixion, but that doesn't mean he fell into the mortal sin kind of despair.
Even when He was on the cross, Jesus' exclamation of "Oh God, oh God, why have you abandoned me?" is not a sign of despair. Aside from being a reference to Psalm 22, asking why God has turned his face away from us is a different concept from losing hope in God's salvation. The former is compatible with the belief that God could save us if he chose to in his wisdom, and the trust that God is in charge and infinitely smarter and more loving than we are.
Despair is a total loss of hope. A total loss of hope means a complete denial of God's ability to save us--either he's not all good & all powerful, or he doesn't exist at all.
And yes, if you're a Catholic, there's no equivocation in Church teaching--it's a mortal sin.