I'm not infertile by any means, and it's unexplained why it took 18 months for my OH and I to conceive our first. Our second was a surprise, as we conceived when our first was 8 months old. Our third is a bigger surprise, as we conceived when our second was 2 months old. haha. It's worth every moment of the wait.
With my first, we TTC and used all the different methods, timing, charting, putting everything on a calendar, right down to scheduling sex so we could try every day, every other day, multiple times a day, 3 days a week, etc. One month, we gave up and decided to take a break. BAM! Pregnant. I think it was the stress of TTC that was stopping it from happening for me, honestly.
And in regards to timing,... with my first, we were DTD once a day. With my second, we DTD once that month and not even during my fertile week. With my third, we DTD a few times a day during my fertile week (I was on the pill, so it shouldn't have been my fertile week... lol) So I wouldn't bother going that crazy with TTC and trying to DTD on certain days. Take a break and have fun! Forget about it for awhile! It will happen when it's your time, and if it doesn't, make sure you've given it enough time to happen naturally because the risks are higher if you are on drugs/medications for fertility, since it can make your body release an unhealthy egg when it normally wouldn't bother.