One more thing from me...
"Doubt" is a natural occurrence in any Martial Art or "Way". Everyone at some point has doubts about what they're learning..it's "effectiveness"... and... their real goals with practice... there appears to be only two finite solutions...Grow through it...or... Move on.
How you handle your own "doubt" is the real challenge. Will you move through it and grow stronger as a result? Or worse...Will you become a dilettante... flitting hither and yon from Waza to Waza until your doubts appear again?
In the old days...Sensei's interviewed students to see if they had the resolve to learn... give themselves over to the Art and "stick with it". Now days... One of the reasons all the Martial Arts have been diluted in spirit is because most folks bail and do not grow past these various personal challenges.
"William: All a black belt means is that you have the potential to be a good student."- Shoji Nishio Shihan