you tend to use low opacity brushes yea? don't do that start with a line drawing, doesn't have to be too detailed, just a sketch that gets all the shapes down use a layer underneath that to fill in the line drawing (like a colouring book) and paint in solid colours, 100% opacity. do all the shading, lighting, highlights etc. once you've coloured in the whole picture, turn off the line drawing layer. the colours should hold up pretty well in the shape of the picture you were trying to paint. go over it once more with the line drawing layer off to clean up any messy bits, and bamb! if you've done all your fundamentals right it should look pretty brofessional just keep doing this until your muscle memory is good enough to paint without training wheels trust me, you'd be surprised how well my pictures hold up without the line drawings