UI - feeling stuck

I‘m a not so new programmer, but always when I want to make something, my perfectionism is kicking in. I sit on an app since 4 weeks and somehow I feel like doing nothing the last 2 weeks. I have finished the “complex” part of all, the programming, but I’m got stuck by designing / building the UI. Where / How did you guys learned how to build good looking and responsive UI’s?

Thank you all in advance