When He (Jesus) rose from prayer, and went back to the disciples, He found them asleep, exhausted from sorrow. - Luke 22:45
The context of this is when Jesus was praying in the garden of Gethsemane before His journey to the cross. He prayed fervently by Himself while He asked the three disciples that were with Him to pray that they not fall into temptation (Luke 22:40,46).Luke's recorcorrding of the gospel is the only one that mentions why they fell asleep. The other recordings simply say they fell asleep, but notice Luke says they fell asleep because they were exhausted from sorrow. Honestly, I always just put it into the explanation of "the spirit is willing, but the flesh is weak", but their flesh was made weak due to the vexing of their souls. Sorrow here is defined as "grief, pain, sadness, heaviness, and grudgingly.
Before we get into the solution, let's point out the danger. The danger in sorrow is that it will, wear you out, destroy your prayer life, and make you vulnerable to temptation. The most important here being that it will destroy your prayer life because if we stop praying we will never see victorious results. When you get distracted from the true intention and get blinded by the current circumstance it will begin steal you strength because you can't clear see an end (aka discouragement). When you get discouraged you will tend to not rely on the source (God) of getting there as you did before. Then when you have become stagnant (not moving forward), you are open to move in the wrong direction (aka give into temptation). (Particularly regarding the loss of vision leading to stagnation and therefore temptation you can look at the story of David and Bathsheba).
Now the solution. Stay focused! On what? Jesus gave them the answer for the three disciples situation not long before they where in the garden. John 16:5-7 says, "But now I (Jesus) go away to Him who sent Me, and none of you asks, "Where are you going?" But because I have said these things to you, SORROW has filled your hearts. Nevertheless, I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you". Jesus knew they would be faced with sorrow even after this moment and while in the garden that is exactly what happened. They were in sorrow because they saw Jesus' own soul being tested and also knowing that this was the beginning of His journey to the cross, or as they saw it, the beginning of the end. In that moment they forgot what the end result of what Jesus was doing. Even after Jesus died they thought it was all over. They struggled to believe what He told them about resurrecting and basically went back to their normal lives (but in Luke 24 you can see Jesus was so gracious to reveal Himself and open up their understand).
God has called us into a life of hope and joy, but we must understand that it is also a life of trials. After all, in 1 Peter 1:6 it tells us to have joy in the midst of trials. It also tells us how to have and keep that joy; stay focused on the hope of the finished work of Christ. Jesus endured the cross by holding fast to the joy set before Him (Hebrews 12:2). Hebrews 12:2 also says that we must keep our eyes fixed on Him.
If we are to keep our eye fixed on Him as it says in Hebrews 12:2 and follow after His example given in Hebrews 12:2, and He tells us to take up our cross and follow Him in Matthew 16:24, then we must know how to live a life of joy in the midst of trials. We must know how to destroy sorrow, grief, sadness, heaviness, or any other form of it, and how to stay persistent in prayer, being convinced in joy of the victorious God-ordained outcome. If a good outcome hasn't come yet then God's not done with it yet, and we must stay alert to press forward with Him to see it through.
Do not give in to sorrow, but behold God's intended victory...
, and destroy that sorrow with joy!