It is impossible to be what God wants us to be when we are in a perpetual state of sadness.  The Bible consistently teaches us to be joyful.  After speaking of the blessing of abiding in Him, and allowing His words to abide in us, Jesus said this:

John 15:11 (ESV)
¹¹  These things I have spoken to you, that my joy may be in you, and that your joy may be full.

Paul learned to rejoice in whatever circumstance he found himself, and even exhorted brethren to rejoice at a time when he was a prisoner at Rome (Philippians 4:4).  Let us not forget that joy is a part of the fruit of the Spirit; thus, we are lacking spiritually if joy does not fill our hearts.  Can we control our feelings, determining whether we are happy or sad?  The answer is yes.  God would not instruct us to be joyful if it was not possible to be so.

Generally speaking, the child of God who struggles with being sad all the time has a focus problem.  If the things of this world are what lay most heavily on our minds, there is a very good chance of sadness often being a companion of ours in life because life is full of trouble.  

Job 14:1 (ESV)
¹  “Man who is born of a woman is few of days and full of trouble.

If, on the other hand, we train ourselves to focus on spiritual things, especially our eternal home in heaven, we can overcome sadness.  Christians often need reminding of this.  

Colossians 3:1-2 (ESV)
¹  If then you have been raised with Christ, seek the things that are above, where Christ is, seated at the right hand of God. ²  Set your minds on things that are above, not on things that are on earth.

It is the things of this earth that get us down.  Do not let those things be the focus of your heart.  Instead, focus on the many spiritual blessings we have in Christ (Ephesians 1:3).  As long as we are faithful to the Lord, these blessings of ours cannot be lost, stolen, or corrupted.  Jesus teaches us this in His great Sermon on the Mount.

Matthew 6:19-21 (ESV)
¹⁹  “Do not lay up for yourselves treasures on earth, where moth and rust destroy and where thieves break in and steal, ²⁰  but lay up for yourselves treasures in heaven, where neither moth nor rust destroys and where thieves do not break in and steal. ²¹  For where your treasure is, there your heart will be also.

This is the divine answer to a troubled heart.  If we lay up our treasures in heaven, our focus will be on things that bring us joy, regardless of the condition of this world, and what is going on in our personal lives.  Daily trauma might bring us moments of sadness and sorrow, but those emotions will give way to feelings of joy if our main focus is on things above, and this is God’s will for us.

Psalm 132:9 (ESV)
⁹  Let your priests be clothed with righteousness, and let your saints shout for joy.

As you wind down for the night, think about these things.