I don't believe Moses' response to God's Call was true humility. I think he was very broken, and had a combination of low self esteem and lack of faith. I believe true humility is seen when someone has confidence in their relationship with the Lord, understands and embraces their identity in Him but chooses to put others first as they wait for the Lord to exalt them.
God reassures Moses by promising to be with him and also by providing him with a sign.
God reassures us when we are called to impossible situations through giving us an anointing to get the job done, by providing us a multitude of counselors, by providing us an assistant if necessary. When we take the time to be Still and Know He is God, we can Hear His Voice. We must choose to listen and accept His provision (both seen and unseen), then carry out what He has called us to do.