If I'm making a rail fence 6 1/2 inches, I cut 7" strips, the middle the exact measurement, the outside ones wider, sew them together, then lop off the extra with the rotary cutter. Not only is perfection overrated, most of us cheat.
Keep going with small projects and give yourself time to get better.
I have about 300 minature blocks and half of them don't quite measure up. So I'm sewing strips around them, and adding a tiny sashing in between. Everyone says they are beautiful and no one has gotten out their tape measure yet.