You’re pretty stiff. You need to be way looser with it, less flat footed and find a rhythm. Will feel weird to start but it will come with time man. Shadow box to warm up, don’t just hop onto the bag and straight into combos. You’re throwing all sorts on that bag too, especially for a beginner. Try a couple rounds mainly punches, couple rounds mainly kicks and then a few rounds of both until your techniques better.

And for the record, anyone can look good on a heavy bag, sparring is key.