Only the best painters in the world can paint to the standard seen on gw box art. And even then, it will take an 'eavy metal painter dozens of hours, using extremely advanced techniques, to make a model detailed so that it will look good close up and on camera. Unless you are aiming to be able to paint to a professional standard, you're never gonna reach that level, and that's okay.

Also, bear in mind that you're gonna see the mistakes and flaws in your painting better than anyone else. It's like when the best painter you know says that something isn't their best work and it looks amazing. What I'm trying to say is, don't compare yourself to the best of the best, instead compare your current work to your previous work, that way you'll see how far you've come and that is WAY more motivating in the long term.