How can I become a better designer?

I'm a software developer and I really struggle with design. I don't seem to be very good at web design or graphic design, or even interior design and fashion sense.

I've recently been doing a terrible job at furnishing a house and it doesn't look very good. I also did a bad job with some previous apartments, so I don't think I'm getting much better by trial and error.

I think I can identify good design when I see it on Pinterest or interior design subreddits, but it never turns out quite right when I try to copy something. I recently created a floorplan in Sweet Home 3D so I can experiment with some ideas. So at least now I can try things out before I buy anything else.

Can anyone recommend some resources that might help me to improve my general sense of design and learn some basic principles?