Hahaha. I write every single kanji I learn with my fingers, honestly, and when I want to test my memory and I don’t have paper, I write phrases in the air. It’s fine. 
Yeah, I struggle with this to some extent when it comes to longer lines, especially because I like detailed work. When we had art lessons in primary school, I once took the entire lesson to draw just the outline of the neck of a vase because I kept feeling like the curve was wrong.
It does get better with practice though. What I found helped the most was writing with a particular shape and set of principles in mind, and making an effort to direct my strokes where I needed them to go. After a while, you start to gain control.
Anyway, since @saibaneko asked,
I vaguely remembered which post that was, so with a bit of searching, I managed to find it:
I definitely won’t claim to have the best handwriting, and I still think that I suffer from writing decent individual kanji without being to produce a page of kanji that go together nicely. (It’s an old problem that my Chinese teacher once mentioned when I asked her what she thought about my handwriting. It got better after I bought a calligraphy course.) However, maybe that’s just by my standards. For what it’s worth, I started formal Japanese lessons this year, and quite recently, after maybe 2-3 months of lessons, my teacher finally said, ‘I just… your characters remind me so much of my grandfather.’ I went, ‘Ah… really? Why?’ She said something about how they were written. I explained that I bought myself a calligraphy course a while back, only to have her tell me her grandfather was a calligraphy teacher. So yeah, I guess it’s decent? 