frequency of screening is down to national guidelines which change from time to time in line with clinical evidence. I was like you in that I had a history of CIN, laser treatment and a cone biopsy back in the early 90's. Annual smears for 5 yrs, then returned to the 3 yrly screening programme. Apparently all my folow up screens were clear - only they weren't as I was diagnosed with cc in 2010.
I have been told that there is a national audit of the results of all previous screening for women who develop cc despite screening - only the audit is massively behind and almost three years on I haven't yet had my results, although I have now asked for them and my Consultant has promised to chase them up for me.
Like you, I now think I should have had annual screening for much longer - but screening is not infallible and also, some tumours are aggressive and fsat growing - although I remain convinved ny tumour would have been found earlier as it was 4.5cms on diagnosis and I find it hard to believe there was no evidence of this at my last smear. Also, despite what our personal opinions are, we're still subject to national guidelines and to get a G.P to go against those can be hard work as even if you are suuessful in getting them to agree to do a smear - the national screening service may still reject the sample or the the lab may refuse to test.
Don't know if that helps any or answers any of your questions. Like everything else these days it all comes down to cost. Given your history of an early diagnosis at a young age I would say there was a need for them to be cautious - but unfortunately as far as the screening programme is concened we are all just statistics.
Good luck with everything, and most of all - listen to your own body and don't be fobbed off!
Lots of love