Knowledge and practice without knowing what went wrong will not help you. This is what will affect your mini painting in order of importance. First 5 are mandatory. Do these first before worrying about technique.
1) lighting - are you in a well lighted place with WHITE light? poor lighting means you cannot see the tip of the brush, and thus cannot paint well
2) miniature holder - do you have a good base to hold onto your miniature? get once of those cork on a tool, or even pin in onto a temporal base.
3) Brushes should cost $20 MINIMUM (not from GW). any ALWAYS buy brush soap. Learn to take care of your brush, load ONLY the tip of the brush. You only need 1 size if you're good (Size 2), but for starters, get a size 000, 0 and 2. Also, get a small flat brush for wet/drybrushing.
4) Wet pallet. No more worries about paint your thins and other rubbish. DOES NOT WORK WITH METALLICS. This is the biggest QOL improvement you will ever make. paints never dry out, paint is always thin, always easy to use, always come off easily.
5) Reduce number of colours. You only need these - Primary(3), main, secondary highlight, edge highlight. Metallics (2) - gunmetal and shiny metal. Gold (2) - redder and yellower. Secondary (3) - per primary. Shades/inks (2-4), brown, black, primary, secondary. You can cover most colours (green to yellow to red) with brown. All in all, 12-14 colours, all very close to each other. Better to have a 2 coloured miniature in 3 shades, than a 6 coloured miniature with no shades.
6) Prime grey. Easy to paint on primary colour, easy to paint on metallics
7) Miniature selection - Get a simple one with large flat surfaces. You don't have to get one that you hate, just choose the simpler option. Box dread instead of furioso. Terminator instead of space hulk ones.
Pic related. What I applied these techniques to.