When I listen back to my own prayers they are almost entirely requests. How do I build real thankfulness in without it sounding forced?

“Bless the LORD, O my soul, and forget not all his benefits:”

David addresses this psalm to himself, which is the interesting part. He tells his own soul to bless God and specifically not to forget the benefits, then lists them: forgiveness, healing, rescue, steadfast love. He is arguing with his own memory. The assumption underneath the whole psalm is that gratitude is not automatic and that forgetting is the default setting for human beings.

“And one of them, when he saw that he was healed, turned back, and with a loud voice glorified God, And fell down on his face at his feet, giving him thanks: and he was a Samaritan.”

Ten men with leprosy call out to Jesus from the distance their illness required, and are told to go and show themselves to the priests. All ten are healed on the way. One, a Samaritan, turns back, and Luke notes that he came with a loud voice and fell at Jesus' feet. Nine received exactly the same thing and kept walking. The difference was not the gift, it was the returning.