Sambrads I am very sorry for your loss.
I do fear however that your fil has unwittingly put a lot of pressure on you because it is his dying wish for you to have another child. If you want to have more children, then it should be because you want to have more children, not because it was the dying wish of someone else.
It is commonly known that stress can cause problems in conceiving, it can delay ovulation and consequently delay periods, so if you are feeling under pressure to fulfil the dying wish of your fil it is entirely possible that this is preventing you from actually conceiving.
I would forget about the wishes of others, put them out of your mind completely, relax and let nature take its course. Whenever you conceive another baby you will have fulfilled your fil?s dying wish, be it today or in 10 years time.
I also wouldn?t be so quick to run to a private clinic just because you have been trying for 6 months. Six months really is no time at all, and it is entirely possible you will conceive in the next six months. Someone used to post a statistic on here about what percentage of people conceive in what time, and it is astounding how many still have not conceived within 12 months. Honestly, although it is very frustrating it is also very normal to take up to 12 months to conceive.
I would however get your thyroid dealt with asap, as this is something that can prevent conception. So if you ar not on any medication already then I would see your gp to see what can be done about it. I would also make sure you speak to him re ttc while on high doses of thyroid medication as it is entirely possible you will be advised to stop ttc until your thyroid levels are normal.