Theophylact of Ohrid

And again going away from them, He began to pray and say the same words as before, so that by this second prayer He might confirm that He was essentially and truly Man, and also to teach us to pray more frequently, and not in such a way that, having said something in prayer once, we immediately abandon it; one must repeat the prayer with diligence.

Commentary on Mark ↗

Hilary of Poitiers

That we may understand what was the cause of His sadness, let us see what precedes and follows this confession of sadness: for in the Passover supper our Lord completely signified the whole mystery of His Passion and our faith. After He had said that they should all be offended in Him, but promised that He would go before them into Galilee, Peter protested that though all the rest should be offended, he would remain faithful and not be offended. …

On the Trinity, Book 10.37 ↗

Bede

And again, he went away and prayed, saying the same words; and returning again, he found them sleeping. He alone prays for all, just as he alone suffers for all. The eyes of the apostles were weakening and oppressed with the nearby denial.

On the Gospel of Mark ↗