I agree, with time this will be easier.
A couple things I would suggest and of course these are just my theories. Keep your back foot on the ground and push off it while twisting your hips. Imagine a boxer throwing a punch. The power comes from using the entire body. Now keep this in mind I am not the strongest slinger, but Ive never been interested in distance so eh. With a short 24 inch sling I’ve gotten about 120-130 I believe... With denser smooth stones and a slightly longer sling I might hit 160 or more but still not great. So take this with a grain of salt.
Second I would try a slightly shorter sling. Until you get used to them longer slings can be hard to get a good pull through.
Third, what type of stones are you using? If you showed them or said it in the video sorry, I havent had a chance to watch the whole thing.
You can see the push off your foot here on this video I made earlier today. Also be aware that a slight step forward can increase the length of your powerstroke giving you more time to accelerate. It also likewise makes it easier to be accurate as your arm is not trying to speed up really fast over a short period of time, which makes you more erratic.
https://slinging.org/forum/YaBB.pl?num=1614975833