Several thoughts come to mind. Get a good physical and make sure there is nothing physically wrong. Depending on your age, several periods in ones life arise that cause us to question what we have done, the oft joked about “midlife” crisis for example. Sometimes a deeper calling from God first manifests itself as depression such as you are experiencing, try talking to God more, fasting as well. Christians are under spiritual attack constantly and the oppression in this age I believe is thicker and more intense than has ever been; the demonic modus operandi is to make every Christian feel isolated as though they are the only one trying to slug it out. I think you’ll find a lot of Christians right at this instant are feeling exactly the same way as you.
The answer is more God and more of one another ultimately; remember Proverbs: “One shall put to flight 1,000, one shall put to flight 10,000, and a three fold cord is not quickly undone!”
One prayer I often pray is that God may use me inspite of myself, inspite of my imperfections, that I might be made an instrument of his glory, a minister of his grace and a conduit of his love. For I have no high station in life, nor have I riches. Every good thing I have comes not of my self but from out of the will of God. I live twixt flesh and spirit,twixt a body of death I yearn to be free from and the promised glorified sinless body promised by Christ.
I’ll die too at some point and will be forgotten of man... yet while I live, I’ll live in the confident hope of that work begun in me, that it will be worked until its completion on that day...when we see HIM(!) face to face.