Watch and pray, that ye enter not into temptation: the spirit indeed [is] willing, but the flesh [is] weak.
Note 3 at Mt 26:41: Our spirits are not our problem. As born-again Christians, we each received a new spirit at conversion (2Co 5:17) that is just like Jesus (1Jo 4:17) because it is the Spirit of Jesus (Ro 8:9 and Ga 4:6). Our born-again spirits are always willing to do God's will. It's our flesh that is the problem.
The flesh, as Jesus was using it here, not only includes our physical bodies but is also describing our souls too (see note 2 at Mt 22:37). God has given every one of us everything that it takes to walk in victory, but "we have this treasure in earthen vessels" (2Co 4:7). That is to say that our spirits, where God has deposited all of His power and glory, are locked inside our flesh.
That does not mean that we cannot tap this divine source. As much as we will renew our minds (Ro 12:2) and act on the Word of God (Jas 2:20), we can experience this divine flow through our physical bodies. However, just as in the physical, muscles have to be used to increase in strength, so our souls and physical parts have to be exercised unto godliness (1Ti 4:7-8).
Prayer is one important way of exercising ourselves unto godliness, and this was why Jesus admonished His disciples to watch and pray with Him.