I hope I can help with this. I know it can be pretty bewildering.
Everyone who has ever had sex gets HPV at some point in their life. It's incredibly common and doesn't have any symptoms. I probably got it the first time I had sex, and you probably did too. The vast majority of people successfully eliminate it from their body within two years without ever having know they've had it, and in the process of course creating immunity from being able to get it again. For some people (like us) our immune systems are not able to clear it naturally. At the present time, medical science does not know why this is. For those of us who are unable to clear it naturally, it stays in our cervix for years and years and still may never do anything. However, for some people (again, like us) the HPV can cause some abnormal changes in cervical cells. From the moment of getting HPV to the moment of having borderline cell changes can take many many years, and obviously it can take even longer for it to get to the point of severe changes or cancer. This is why the smear test has been amazingly effective at bringing down the rates of cervical cancer, because it can detect changes before they get the chance to develop into cancer and deal with them, thereby preventing cancer from ever developing. So, although I know it doesn't feel like it at the time, the length of time that you've been with your partner or how many sexual partners that you've had really isn't a significant factor - it only takes one and that was probably in the early days of becoming sexually active.
It might also help to know that contrary to popular belief and what you might read on some American sites, you and your partner won't be continually passing HPV between you, ping pong fashion! As I said, most people have no trouble eliminating HPV and there is no reason to assume (and actually it would be one hell of a coincidence) that your partner is someone who is unable to trigger the appropriate immune response. The likelihood is that he has done this and is now immune to that strain.
It may also help to know that there is a current theory in medical science that treatment for cell changes (such as the LLETZ) not only deals with the abnormal cells, but also triggers the appropriate immune response in the body, clearing the HPV and creating immunity from that strain. This doesn't seem to happen in 100% of cases, but it happens in enough that it can be called the 'norm' at the moment.
Dons, I had the same as you - always clear smears and then cancer. When they reviewed my 'old' smears after I'd had cancer, they discovered that my previous one (three years before I was diagnosed) had actually shown abnormalities, even though it had been reported as normal. They are still unsure about how this happened and it was difficult for me to hear because now I have the knowledge that I could have potentially prevented cancer ever developing in my body. All I can say is that in the vast majority of cases the system works and abnormalities get picked up on. But the test, like everything else I guess, isn't perfect and very occassionally things get missed. This was particularly the case with CGIN abnormalities or adenocarcinoma (as I had) because the smear test was specifically designed to detect abnormalities in squamous cells (because they cause 80% of cc). But now they have the new 'brush' smear test, I think that will help pick up more of these kinds of abnormalities at an earlier stage. Not much help for us, I know, but as you've pointed out, sometimes you just have to put things down to 'life'.