I think this is one of those strange phenomenons that every artist experiences. I've struggled with this myself throughout my career. it seems that there is a pureness to the beginner's attitude when first learning something new that encourages certain creative resources that perhaps get restricted when we start "trying" too hard. So in the beginning, we don't want or expect any kind of result. And so this mindset allows a lot of mental freedom. We are just having fun and exploring something new. As we get better, we tend to begin investing ourselves more and more into what we do. We begin to get attached to the outcome, the performance/drawing/whatever, and as a result, we're putting a completely different energy into the creative process.
So i think what you're going through is very normal and part of the spiritual process of becoming an artist. You need to navigate the waters of overly investing yourself emotionally in your art all while continuing to do it regularly. If you can master the art of non attachment, then you will bring the best mindset into your creative process. It's much easier said than done though! I struggle with this daily. There is so much stress that goes into being a Disney animator at times. The level of quality that is expected, deadlines, etc...can really take you out of the "zone" very quickly. I find that meditations can help with this. Or, your creative process can be a mediation in of itself. Just notice that when you sit and work for a prolonged period of time, you'll start falling into the "zone". Mental clarity and focus becomes more sharpened, you become relaxed, and the art flows better as a result. Hope this helps!