I was as well doing 1 lesson a week for about a year, half of them got cancelled due to for circuits it needs to be near perfect.
If a lesson is cancelled use it wisely and revise for your airlaw and other 5 exams. Alternative stick with it and as your self employeed be generious on your time, maybe when a good week of weather is in sight try to book as much flying as possible, do it after work ect??
Thats odd my instructer used the poor weather days for circuits as it was good practice and the vis was too crap to go outside the circuit.