Star quilts are actually quite easy. I recommend if strip piecing to start with something simple like 3 diamonds in a row instead of 7.
Do you have a 1/4" foot - all your seams need to be the same size.
I never use strip piecing for large star quilts though and I use 2 1/2" diamonds. I use to use larger or smaller but I like the flexibility of the 2 1/2".
I also recommend only using good quality cottons for the diamonds, no cheap fabric, not even for practice, it will never turn out just right.
You did a great job for your first, my first only had 7 points instead of 8.