You can have light inside, The Holy Spirit, and still have darkness in your life. But if you have the Holy Spirit it then is a choice. Darkness can mean fog, thick mist, gloom. Even though we have the light we can walk around in gloom and despair. This isn't how God wants it for any of His kids. It is our choice.
In Mark 7:20-23, Jesus lists the things that are evil that come out of a person's heart. The second to last on the list is foolishness! It is up there with envy, immorality, greed, lust.
In Matthew 7:26 Jesus tells us what foolishness is: if you hear His words and do not follow them.
We Believers are foolish if we allow despair or gloom to color our worlds. We cannot shine for God then at all. It is up to us to keep our inner selves right with God. If we do that, then God will shine through us and touch others. If you need to forgive, forgive. If you need to deal with some anger, ask God and He'll help you. If you have a burden to heavy to bear, go to Jesus and learn His ways. His ways are "gentle" and "Humble" and you will find rest for your soul (Matthew 11).
So I am not misunderstood, this doesn't mean you put on a happy face and smile when you're crying inside. We aren't called to fake it. We are called to be honest. But in that honesty we have to let God work. There is no reason a Believer cannot live wholly in God's peace, live always in God. That is what we are supposed to do.
Jesus says, "I am the vine, you are the branches ... Live in Me." (from John 15)