To measur whether it is humility or depression, check whether you are advancing spiritually or going down.
The more one makes spiritual advancement, one feels humble and as one's devotion is superficial that much he feels that he is a great devotee.
Theoretical knowledge can not touch heart without Humility
Substance of Bhakti is Surrender and Humility.
Arrogance repels the Lord; humility attracts the Lord.
Even if you have done no wrong, no mistake in the last 50 years, remember the mistake you did 51 years ago and repent for it and become humble.