I am sorry you are having such troubles, I think we have all been there!
1: I highly recommend starching fabrics before cutting. It provides oomph to the fabric and makes cutting more accurate, at least for me, YMMV.
2: When measuring and cutting, be sure the line is on the fabric. Bonnie Hunter has an excellent video on this.
3: I always make a practice block, and adjust accordingly if needed. I then make a notation of what is needed so I don't forget. Each quilt gets notations on a legal pad until I am done with it, in case I have to pause for a while.
4: Pinning! I am a firm believer in pinning, especially if there are smaller units involved. Yes, it takes more time, but IMHO it’s worth the effort. I am a methodical quilter, I set my machine to sewing speed to middle, not too fast not too slow and I guide my fabric, and let the feed dogs do the work.
5: This is what works for me, YMMV, and I am sure others will chime in with more tips and tricks.
You’ve got this!