Maximum we were in a very similar position to you I think, and I do understand how you feel. We started trying for No 2 when DS would have been about 15 months (so sounds a similar age comparatively to your son?) And I know what you mean - our friends with children the same age started trying for no 2, then got pregnant, and in so many cases now have their new babies (in fact some of them aren't so little any more!).
In the end we were trying for 19 months and had been referred to the fertility clinic. (I should probably say, although this isn't necessarily helpful, that we actually got our first positive pregnancy test this week so very early days but a darn sight further than we've got so far in the 19 months we've been trying!).
However, I had been trying to come to terms with the idea that we might only have one child and it was really taking some getting used to.
Have you thought about going to see your GP? Secondary fertility is hard because in many cases (and certainly in ours) we didn't really have any trouble getting pg the first time. In fact we got pg twice but miscarried the first time. But then you just don't anticipate any problems the second time. We saw our GP after 1 year with no success and he sent us for blood tests (for me) and a sperm analysis for my husband. When we got those results back, the sperm motility was very low and they referred us to the fertility unit We had had further investigations there and just 2 weeks ago went back for our follow up appt there. We were anticipating that we might be looking at IUI or IVF but in fact they prescribed us Clomid to try for a few months before IUI/IVF. (although at the moment I am hopeful we won't need it). All the tests we have had done so far have been done on the NHS (although sometimes the waits are a bit long) but we would have had to pay for IUI or IVF if it was needed.
I felt very sad about having to 'bring in a medical person' but in a way it did help me to feel that we were progressing towards our goal.
One of the tests that we had done as well, was a HSG x-ray and (like a few of the exploratory tests) there are reports that you can be more fertile after those tests (we got pg the second cycle after the HSG).
It is probably worth getting some investigation done as you may find it is something relatively easy to improve. After our first semen analysis, hubby stopped drinking pepsi and things and started taking conception vitamins to help his sperm and the second analysis he had showed a great improvement (which could be coincidence or not).
Hope that helps a little bit. xx