What was Their Problem?

Still reading Exodus: Moses has just led the Israelites through the Red Sea and the Egyptian army has been destroyed by the Lord. He has delivered the Israelites miraculously out of the hands of Pharaoh. From that day on, the people trust God and believe Him, never complaining, but confidently trusting in His provision. What? That's not the story? Oh, yeah. Not three days later (Exodus 15: 22-24):

Then Moses made Israel set out from the Red Sea, and they went into the wilderness of Shur. They went three days in the wilderness and found no water. When they came to Marah, they could not drink the water of Marah because it was bitter; therefore it was named Marah. And the people grumbled against Moses, saying, "What shall we drink?"

Three days ago God struck down every first born of the Egyptians, parted the Red Sea so they could walk through it, and destroyed their enemies. Today the people have no water and they instantly start complaining. What was their problem?

The footnote in my study Bible says this: "They [Israelites] do not yet trust that the Lord's presence with them is sufficient for their protection and
provision. . . " (p. 170, ESV Study Bible).

Grumbling and complaining is probably my biggest struggle, and I often wonder why. What is underneath that? I have seen the Lord change the lives of others. I have seen Him work in my life. I have seen Him answer prayers and do great things. Yet if I'm too cold or too hot or hungry or inconvenienced. . . what is my problem? Is it that I don't trust that the Lord's presence is sufficient for my protection and provision? I will keep praying about this and seeking both understanding and the discipline simply to not complain--and I would appreciate any prayers from any of you that are reading, too, because, if you know the rest of the story. . . it wasn't pretty for the people of Israel. There were big consequences for their lack of trust.