Like everybody said, it's all about practice and patience, but it's also about common sense. If you can look at your replay a couple of times and see the flaws by yourself then you should be able to improve decently, or at least if you listen to other people's criticism over your replays.

Also, a time spent on a good replay doesn't necessarily take days. It depends on how you work with it ( in which case no matter how good you are, it may take a lot of time) and how much joint knowledge you have. At first it will take a LOT of time to find out what each joint does in a specific situation, but after you get the hang of it everything becomes much easier.

Also SP and MP are a bit alike, but they just serve different purposes. If you are experienced with joints and movement from SP, then you can use that to win easier in MP. The other way around is also possible, but less likely since online you are generally more concerned with winning than how you look in the process.

Practice more or go to a school

Patience is the first thing you should need.If you don't have patience,you can't practice which is the main thing you should do.Everybody needs practice.No one is perfect.
I am no pro,but I still remember how I sucked at green belt and how I improved somehow in a not so long time.

hehe i watch those videos on youtube and i feel like such a noob to the point where im like "daaamnnnn i suck at this game"

A pinch of patience, and a bowl of practice will have amazing results. Just keep at it for a long time, and you'll get there in no time.