Such problems aren’t uncommon, but they are readily remedied with practise. 😉
1. Try and ease off pressure on the front of the plane and keep the plane sole in contact with the board as you approach the end of each planing stroke. This should help keep the plane’s toe raised as the plane reaches the end of the board, while keeping the plane referenced on the board will tend to keep it on the same plain.
2. Set the plane blade square to the sole. With the blade adjusted it proves possible to focus on keeping the rear handle vertical during use.
3. Check the edge for square after each planing stroke.
4. Adjust the horizontal angle at which you offer the plane to the work. Taping a lollipop stick to the side of your plane and matching it’s alignment to the vertical plain on the side of the work piece can help maintain plumb.
5. Reduce the set of the blade and this will tend to minimise discrepancies. Smaller discrepancies are – in turn – far easier to correct.
6. Mark a longitudinal reference line on each vertical face of the board as a ready guide and means of determining a square and straight edge. This should help and you’ll be surprised just how soon you reach a point there the need for a secondary line becomes obsolete.
I hope this helps 🙂