You are not going mad at all - you are being quite sensible. My husband just died three weeks ago, and it was such a lot of stress off me that his funeral had been arranged and paid for four years ago! All I had to do was phone the Funeral Directors and tell them he had died. There was still quite a bit to do, as there was a lot of family input into the service etc. - but the things I would have found awful - choosing a coffin, a gravesite, music, arranging flowers, transport to the funeral, having it announced on local radio etc. etc. - all done. His headstone was also pre-ordered and paid for, although that won't be able to be erected yet.
I am thinking that if I can afford it, I might do the same for myself - because I only have one daughter, and I don't want her to have all the stress of having to arrange everything when the time comes, and wondering what to do. Most certainly, I will be leaving instructions as to what to do.
Beate is right about giving someone Power of Attorney to see to your funeral wishes though rather than just put it in your Will - I don't know about over there, but here, a Will would not be read until after a funeral - sometimes if it wasn't expected to contain anything out of the ordinary, it could be several weeks later! I used to work for Solicitors, and we always advised people never to put anything like funeral or burial preferences in their Will, as it would be usually too late by the time the Will was read.