Rudebwoy,
I think conducting your first ever ‘service’ for someone close to you would be hard. Doing it with those difficult circumstances would be particularly tough. Personally I would pass the ‘service’ on to a professional – but of course get involved in the eulogy etc. If you really want to DIY it then i’d still be tempted to get involved with a professional who will be able to advise. I would also ask them to be there in the background just in case you can’t do it on the day (which nobody would judge you for). The worst funeral I have ever been to the person reading the eulogy broke down, seemingly taking the minister by surprise and nobody seemed to know what to do…
If for any reason you don’t want a Humanist but still want a non-religious ceremony there are other options, the funeral director will be able to point you to the right people.