Unexplained here too - all tests normal, and a textbook regular cycle, but nothing happened the natural way
despite 3 years of trying (age 35-7). It's maddening not to know why I wasn't conceiving. However, once past a certain point in testing all roads will end in IVF anyway, and if the treatment is going to be the same, I follow the logic that further investigations are a bit academic.
FWIW I had the AMH test privately once the NHS tests had run out of ideas, together with an antral follicle count and first consultation at the fertility clinic. The idea was to gain extra information to help decide how much longer to wait before IVF, and discuss options/IVF so I knew what plan B was. No clues from the extra tests as to the reason I never conceived - ovarian reserve was textbook fine and dandy too - but it was def worth doing for peace of mind and it's reasonably non-invasive. The cost was about £200-250 all in, 6 years ago.
I think it's pretty natural when you're "unexplained" to fear the worst and just know in your bones the problem is something intractable that IVF won't overcome. I was absolutely convinced that I had an implantation issue, particularly after the IVF first cycle ended in chemical pregnancy. However... The facts are that we had 12 eggs (with mild IVF), 7 fertilised, 4 made it to blastocyst, and embie number 3 (frostie no 2) stuck. He starts school in September.
Your cumulative odds with unexplained are pretty good over a course of 3 IVF cycles (sadly the chances of success recede after 3 or 4 failed cycles, but you're a long way off that yet). I believe thinking the worst is how we try to protect ourselves, during investigations and treatment, from the rollercoaster of raised and dashed hope, because it's the hope that gets you every single time.
Hang in there and good luck with the IVF.