What feeling should we have in mind while chanting the maha mantra?
Hare Krishna! Thanks for your question. Actually the most important thing when chanting the maha mantra is to hear the mantra. Other feelings and thoughts will come naturally as we begin to hear the Mantra and relate deeply to the Lord as He appears in His Holy Name. It is not necessary to try to feel or think about anything. Just chant and hear.
According to Lord Chaitanya the most important feeling ultimately, is humility. When one is thinking himself lower than the straw in the street, more tolerant than the tree, offering all respects to others, wanting none for himself/herself, in such a state of mind one can chant the Holy Name constantly. So considering that one is a tiny soul, in a vast and complicated universe and that each soul really has no control of anything except his/her ability to worship the Lord will help us to remain humble and sincerely call out to Krishna to accept us!
I hope this is helpful.