sorry to hear about your loss Fluffy.
i look after a lot of cats, and one of my favourites, Smudge, has a large growing tumour in his jaw/throat. the vet and i are monitoring closely, and at the moment, Smudge is still in blissful ignorance - he isn't in pain, nor does it seem to affect his breathing and eating - yet.
however, it will do in a month or two's time, and when that happens we will have to do the right thing for him and help him out of his suffering.
as i keep so many cats, mainly oldies, this is a decision i have to regretfully take regularly. but i am still thankful that i've had the chance to enjoy their company for the time that we shared together.
i know it's not the same, but i'm sure the right pup will find it's way to you before long.
as A Guy mentioned above, try to think of the good times, and not the last few weeks when you think about Simba.