Take your time. You're building up a skill. It's better to practice slowly & well on a few exercises than fast & wonky on a lot.
A good antidote for problem days (like today): Ditch the shapes and practice lines. Try to get a few good:
- straight lines (connect point a to b), and
- right angles (practice crosses)
You can use the corner of a sheet of paper to judge the right angles. Each of the four angles in a cross will be right angles (so they should all look indentical). Aim for that.
When you come back to squares and ellipses, an improved instinct for 90° angles will help.