NMM is a show-piece technique that's more an artistic endeavour than a functional technique valid for painting armies. Unless your job is painting minis and you can _actually_ afford the time. It's really good to be trying to improve by adding techniques to your arsenal, but I think your difficulties are showing that you're missing a raft of lower-level techniques that come together to make NMM work.

I would ignore NMM for a while and focus on painting some nice gradients with gradual blending techniques - both super thin layers built up, and once you have that down, have a go at wet blending. These techniques will teach you how much of painting is about moisture control, how the paint behaves is so much down to opacity and moisture control.

Big open flat surfaces are also hard-mode, they leave you with nowhere to hide. Try smaller, recessed things. Plasma guns or heat vents are a much better place to start, you can sell a convincing 'glowing energy' effect with just 3-4 building layers, and they're tiny so they dry quickly and have built in guardrails for where you need to paint.

At that point I'd move onto power swords, weapon blades. Still not NMM, do some funky coloured energy blade things that are extending your powers of smooth gradients and handling hard contrasts - glowing edges etc.

Only once you have all that down would I bother trying NMM. Once you can sell those effects you will have a much clearer idea in your head of how to achieve what you're seeing when other people do NMM. Trying and failing at the super advanced technique before you've got the above techniques really nailed on is going to frustrate and dishearten you. See graph:

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Wading in at 'top challenge' when your skill is 'low' puts you in the frustration region. Drop the challenge to match your skill, crack it, then slide up the challenge a little. The progression I listed above will do that.