The Hebrew word for contentment is ya'al. In the Bible, part of being content is being willing or pleased to start something that requires our submission. Our determination to undertake a course of action no matter how hesitant or reluctant we are. Or afraid.
Doing what we need to do, even if we don't want to do it. We have a resolved determination to do so no matter what challenges or difficulties lie ahead. We "Damn the torpedoes. Full speed ahead." Knowing we are self-sufficient.
Not self-sufficient as in we think of it today. It isn't so much about thinking we can provide for ourselves without the help of others or independence, so much as it is a walk of faith after taking a "leap to faith".
If we've had any life experiences that were difficult, challenging, almost soul and will breaking in their scope, we know Who brought us through such hard times. We felt His arms around us. His love carrying us. Providing. Keeping. Guiding. He was and is building up our trust in Him and what He will do for us based on past experiences.
Like Abraham sacrificing Isaac. It was nothing short of full trust in God providing whatever was needed at that time and place, based on His doing so in the past for Abraham. Abraham knew Who was in charge and that what was about to happen was fully for his benefit. He trusted God to resurrect Isaac if it came to that need. He was content with what God was going to do for him.
In Phillipians,I don't think Paul was talking about self-sufficiency the way we define it. I think he was talking about learning through these life experiences with God to accept where we are as sufficient. Sufficient to draw closer to Him. Sufficient to continue building up that relationship of trust and love. Faith in a God Who always meets our needs and is always working for our good.
Paul had learned not to count on himself or others to fix what was wrong in his life. He was content to be invited to trust a little deeper. Move a little closer. Change just a bit more into the Likeness. He had learned to accept, and even be thankful, for his own lot in life. It was perfectly tailored for him.
And it was enough for him.