The next time you don’t “feel” like a Christian, do a gut check. Go to God and ask, “Have I sinned against you?” (See Psalm 139:23-24.) If you determine your bad feelings are a result of sin, ask God to forgive you. And ask the Holy Spirit to help you go on walking with God.
And think about those times when you’re on fire for God. What are you doing during those times that gives you joy? You’re probably reading your Bible, spending time in prayer, hanging out with Christians, going to Bible studies, telling others about your faith.
These are the kinds of things you need to do regularly and consistently. As you do, I think you’ll experience fewer and fewer roller-coaster rides and that fire will burn stronger all the time.