We spent over 18 months actively ttc, had every single test under the sun, with no reason for not getting pregnant and finally conceived naturally, and I'm now 12.5 weeks pregnant with our first. OP, don't lose heart, some people take more time than others, altho I know that's no consolation when you're trying and nothing is happening.
My long story is below, not sure if it helps, but might give you some insight as to the ups and downs. My biggest takeaway from all of this is that you and your partner really need to be on the same page about what you want, and will and won't do, and you approach things. We had some pretty soul searching conversations when we first started trying about our opinions on stuff like our approach to IVF / donor eggs, what tests we would and wouldn't do, how long we'd keep trying, what if we got pregnant and we found out there was a problem with the baby etc. We kept revisiting this as we went through the 18 months, and tbh, it probably helped keep me sane when I was losing it.
Our road to get here was probably like a lot of peoples's who take time. The first 6 months was all "ooh, isn't this fun" lots of sex, trying to time it around ovulation and day dreaming about being able to pack away that seasons clothes and buy maternity wear.
After 6 months things got a bit more realistic; I was 36/37 and didn't think I had time to lose so I had the initial round of tests to check I was ovulating, that the egg reserve was good,that my partner's sperm count was fine etc. At this point I also went for the HSG (the tube flushing procedure) and that was a real smack round the face as the clinic staff (who had NO patient skills) basically went "oh, you're here because you're infertile, right?" in my head, I wasn't actually infertile, I was checking to see I wasn't!
The next 6 months were spent religiously peeing on sticks, checking cervical mucus (and sharing way tmi on the internet..) altering diets and starting to see an acupuncturist. It was also the start of me really starting to lose it mentally as everyone around me seemed to be getting pregnant, and whatever we did, it just wasn't working.
As a result, it culminated in us starting at a fertility clinic where we sent for another round of genetic and other tests, scans, endometrial biopsy, HSN and god knows what else. Again, the tests came back fine with no identifiable reason, and the clinic wanted to push for invasive surgery "to check that nothing was wrong" (even tho they couldn't find anything) and I started to get cold feet about their approach, and they were really pushing us for IVF which was not something we wanted to do. We were debating IUI (turkey basters all round!) but at our ages the stats for success weren't good.
For various reasons we decided to walk away from them; I had a total mental meltdown (bubbling snot and all 
) in our kitchen one day with the OH and we decided that was it and that if it happened it happened but it was time to start thinking and planning for life without kids, as the whole thing had left us hanging. Two months later I got pregnant.
Who knows why; my mum's theory is "oh, you see, I told you to relax and it'd happen!" (coming from someone who'd also had problems conceiving is the only reason I physically restrained myself from smacking her) it could be all the messing around from the invasive tests helped (there's a theory that an endometrial biopsy helps an egg embed) it could be I upped my really low vitamin D levels, or it could just be timing (average time at our age to conceive naturally is 12-24 months)
There's some interesting reading and stats in the books 'The Impatient Woman's guide to getting Pregnant' by Jean Twenge on the average time to conceive.