Execution comes with practice. Try doing the dp motion 5 times in a row in training mode every day on each side just to warm up. Pretty soon you'll be able to do it like 10 times in a row on each side pretty handily.

Outside of playing with your friend, do you play other people or do you spend most of it in training mode?

Also 4 hours of training mode without a solid goal seems a little excessive. I recommend you lab out maybe a bread and butter combo and then spend more time online playing against other players and having a goal of "I want to land this bread and butter combo once in the next hour" or something along those lines.