At first "good" ist relative. When I watch dancers I seperate between technique (including speed - fast and slow -, movement variation and sense of tact for example) and the style (because, even if technique is brilliant, maybe you do not like it).
One other thing you need to consider - videos and club dancing can be really different. Because the mood is different. For me it's a huge difference to dance anywhere or between friends, that give their best, too. (I need to say that only because of the mentioned youtube videos :) )
And now - the actual theme - how to get better. Or how I get/got better. I'm never satisfied. I didn't learn it by standing in front of a mirror. A friend showed it in the club and from this point I always danced only the club for several years. Now I dance at home, too, but not for training. It feels like a inner force, when I hear good music - I just need to dance. It doesn't matter if it's industrial, wave or just bouncing the hips.
To get better - you need to feel the music and the passion and then the improvement comes by itself. (but at first you need sense of tact)
Never be angry if people critize you, as long it's constructive. If people have tips and ideas for you, you shouldn't reject this optinion at first site. Take a minute, think over it - maybe it's a bit true? - and then decide, will you consider it?
Next is - To make sports beside dancing helps to improve your speed and strength. The endurance improves with your fitness level.