I can tell you with confidence that physical color mixing itself is a really small part of what makes a good traditional artist, and I am indeed talking about realistic paintings. All the art fundamentals are exactly the same, wether you do digital art or traditional oil, there are just some technical differences on top. I have been learning digital painting for a few years and the hardest things to learn about color were identical to traditional painters. In fact, after years of learning digital painting and about colors, it only took me a couple of days to understand and perform traditional color mixing with oil. The difficult part is knowing what colors you need, not how to get there (mixing, using the sliders, etc.)

And just to add a small bit here: digital artist also color mix all the time and need to know how it works, the difference here is that mixing is additive instead of subtractive.

  (1) go to a GP (Hausarzt) and explain my situation so they fill up a report and confirm that the issue is indeed psychological and not physical,  

  (2) go to a therapist that works with public insurance so they can write another report, your therapist might refer you to one of his/her colleagues, and  

  (3) email five therapists that work with public insurance and ask them how long their waiting list is. They likely will all tell you it's longer than 3 months, which is what you need to hear.