this probably sounds silly, but what really helped me a lot was the advice I saw on youtube from Holice Turnbow (Horice Burntoe.........???? dont remember the name)
You cant sew a straight line unless sitting right in front of the machine!
Getting up ironing and sitting down again I did often not take the time to sit straight in front of the machine.
Also as already said: dont look at the needle, look where you go.
Granny