I know this sounds like frustrating cliche from skilled people but it's actually really true. The other helpful thing is to find an art mentor who you have good chemistry with and who will work with you on improving at a reasonable pace that suits your level. My friend started tutoring me last year and it was like witchcraft-voodoo how I've improved since then. The more you draw, or paint, or whatever, the better your eyes get at determining where things need change or improvement.
In the words of the great Confucius, "It does not matter how slowly you go, as long as you do not stop."