Gosh, I'm there with you!
I'm new to it, did a machine table runner in a class, am trying a hand-pieced sampler on my own and am still on the FIRST BLOCK! I'm on my 6th try now. <sigh> At least I can say I'm learning from it and expect to improve.
What I'm finding might work for you:
1. Measure, cut, then re-measure.
2. Draw a thin line to follow for every seam allowance. re-check the measurements.
3. Unless you're strip piecing by machine, check every piece as you fasten it to another, so you can rip before it causes problems across the whole quilt top.
4. I was taught to iron seam allowances under the darker fabric, but that gets bulky where four corners meet, especially when it's on an angle. Use your judgment rather then blindly following a rule, but consider the consequences when you make your decison.
5. Line up your seams when sewing two pieced pieces together, rather than lining up outer edges.
6. Basting rather than pinning works better for me re keeping the pieces in place while hand sewing.
7. Decide which imperfections you can live with. Most people won't notice or care about small mismatches in seam intersections.
8. Seek feedback from others. Sometimes there's another method that works better.