Yes, happy people can sometimes have periods of sadness and sorrow. The Bible tells us that grief and sorrow are an unavoidable part of this life. I have learned firsthand that some Christians think and believe that other Christians who are going through a season of sadness are somehow unspiritual!
I strongly disagree with this perspective. Especially since the Lord Jesus experienced times of sorrow and sadness. Paul tells us in 2 Corinthians 1:3-5 tell us that our times of sorrow and all that we experience in that season will one day be used to comfort other people and help them.
Times of sorrow are beneficial. They can bring long-term benefits and bring things into clearer perspective. These times also help us learn how to make wise choices and decisions that will bring rewards here and in the future.
When we are sad, it is important to realize that happiness is not found in having the ability to ignore pain, but it is in knowing how to successfully overcome the emotional pain that comes with circumstances. Please know that God always comforts us in our loss and pain. Our problem with that is, we don't like how He does it. Our desire is that He would comfort us by changing the circumstances or removing the pain but that is not how God operates. While He does not change our situations, He moves through them with us and actually - He moves through them within us. That is even better!
Remind yourself that God will not waste any trial you go through. I have had to remind myself of that daily over this past year, and most particularly over this past 4 months. God will use all of our hurts, pains, trials, and misery for His glory and for our good. Yes, for your good. Hard part is, we don't usually like the "good" either because it is not focused on making us feel better. It is focused on changing us to the likeness of Christ.
Believe that one day, God will completely deliver you from these afflictions you are now enduring. He will wipe away every tear and remove all sadness and sorrow from your life. God is always good! He is always faithful and worthy of our trust and confidence.
Do not buy the line that Christians are never sad or unhappy. My goodness we grieve and sorrow like everyone else! It does not mean you lack faith or hope or trust in God, but if you find yourself staying in the pit of sorrow you had better do some re-evaluation of what your heart is focusing on.
Keep Him always in your sight, keep looking up and trusting Him, no matter how bleak it all may look.