The Bible says "contentment is a great gain." We would always have enough when we learn to appreciate the little we have.

I agree with your point, but this isn't a correct quote. 1 Timothy 6:6 says, "But godliness with contentment is great gain." (ESV) Meaning that godliness is the gain, not the contentment. Contentment is the fruit of faith, knowing that, as you said, God will provide what we need at exactly the right time. So the gain here isn't material (though it can be), the gain is the godliness itself - the state of being closer to and more like our Creator.

Bit of a side note -- I don't think it would be a stretch to think of godliness here as love because, "The one who does not love does not know God, because God is love." - 1 John 4:8 (LEB), and "And now these three things remain: faith, hope, and love. But the greatest of these is love." - 1 Corinthians 13:13 (LEB).

When you lack contentment, you develop hatred for another person and block your good relationship with them.

I like this way of thinking. Lack of contentment and envy are closely entwined, envy leads to hatred (or in some ways is hatred), and you can't be in close relationship with God or with others when you have hatred for another. This also limits your ability to treat yourself the way you ought to, because I don't think you can feel any better about yourself than you do about anyone else in your life.

Great read. I'll be following!