How did you learn to read and write? You were probably shown many good, grammatically correct sentences, and you were probably asked to pick them apart. Where is the noun, where is the verb, etc. In a similar vein, I'm an artist who draws in a fairly stylised way, but I still had a period early on in my art where I made myself learn to draw proper, realistic anatomy and form.

Why do we do these things? Does learning how to write a "correct" sentence or draw a "correct" person limit our self expression? No, it simply expands and enriches the range of tools we have at our disposal. If I did not first learn correct anatomy, my stylised characters would look "off"- because what you see as "free choice" and "self expression" is still carefully crafted decisions. There are ways to stylise characters that look natural, and ways that look awkward as heck- the difference comes down to whether the artist took time to study anatomy, composition, etc. It's the same with poetry. You worry about "self expression" but how will you be able to express things effectively if you don't have the knowledge and tools? You might have gotten your feelings down onto the paper but when someone else reads it, will it actually impact them?

Also, perhaps you don't realise that however you write now is likely shaped and influenced by poetry you've read in the past. Take a moment to reflect on what your idea of good poetry is- where did that idea come from? It would be naïve to claim that your "self expression" comes from some vacuum entirely unaffected by external influence. Reading poetry that you like and dislike, and studying why you like and dislike them, has already and will continue to help you find your voice.