Firstly, 17% is not 17% of everyone that studies A-level maths. It is 17% of the people who take the test. In my Collage only further maths or A/A* second year maths students could take it. In my case those who could take the test had to, but in others it may be optional, so students who enjoy maths or are applying to a maths-ish subject for university would be more likely to take the test. This means that there are many people you would have done better than, that just did not take the test in the first place.
Secondly, the SMC measures some very specific (and often unnecessary things). Yes, there is problem solving. There is also geometric intuition (though granted, less this year), arithmetic skills, and still a lot of pattern recognition and remembering various tricks. You could be an amazing problem solver, who gets mixed up with your multiplication tables, or something of the sort, and get a silver, a bronze, or nothing at all. I know some incredibly intelligent people, who are likely going to Cambridge or another top school, and only got participation.
I barely scraped a silver last year, but I knew I wanted to do better, and have been preparing over the summer, just looking at as much maths stuff as I can.( I was also preparing for admissions tests, I am not THAT devoted to the SMC. 🙂 ) This year I got a gold and top in school. People sometimes treat problem solving as the same thing as IQ, as if it is unchanging. However, it is defiantly a skill you can grow by just trying more maths.
Don't beat yourself up. Defining your abilities by the SMC is just as pointless as defining your abilities by your A-level results. They both test a very narrow set of skills, on one day, on one paper. And a silver is great.