I had to learn that what existed in my mind was very different to what I actually feel good and look good in.
I never feel that i quite know what suits me though. It's tricky. I spent years in a world full of skinny jeans, where almost everything was tight fitting and uncomfortable, and even though im on the thin side i hated how these clothes felt on my body.
Now the looser trend is here, I look swamped by things. I can't win.
So i learned to simplify. Like paring everything back to absolute basics and i do well there. I have a short waist and so high waisted clothes look wrong on me, so i stick to a few things that i trust and dont get swept away by trends.
I had to learn that we have too much choice when shopping, and this made me more confused and distracted. A trillion page scroll on ASOS is not my cup of tea! I stick to three companies now and rarely go elsewhere. It has saved me a ton of money and my head isnt easily turned with flashy new things.
As for my looks, I had to accept that I have very porous fuzzy hair. It is healthy and i take care of it but it always looks decidedly off trend, unless I go to the orchestra or an art fair 
I longed for Pam Morrison style straight hair when young and did everything in my power to change it. Now I just go with the flow, I will never have heavy straight hair that i can just wash and go. It will always be fluffy, defy gravity and look a bit awkward, and i got tired of feeling crap about it.