I'm posting this in desperation and just really unsure what to do, or even if anyone can offer any advice.

My gorgeous Stafford, Tess, was diagnosed on friday with osteosarcoma :(

She'd had smelly breath for a week or so, and so I went to clean her teeth and discovered a large lump at the back of her mouth on her palate right next to her back molar.
Took her straight to the vet, who suggested surgery to remove and further investigation including biopsy.
Prior to this Tess was pretty much almost her normal self, if only a little quieter than usual. I'd put it down to the heat and just her age more than anything...
Results came back on Friday and sadly the results have shown that she has osteosarcoma. We do not at this stage know how extensive the spread is or if it is even in her bones... however our vet feels that it most likely is, as that back tooth is quite wobbly.
There are a couple of routes to go down, one of which being further investigation to see how far the spread is, and then surgery to remove affected bone, plus radiotherapy/chemo. We have decided already that this is not a route we wish to go down, because of how invasive the surgery would be and due to Tess's age. The vet seems to think that it could be only a matter of weeks or months.

The lump was cut flush with her palate, though the vet thinks he has not gotten all of it out/off. There is now swelling in the same place as the lump, but not sure if this is from the surgery of if it's another growth already.

We know that the decision has to be made to have Tess pts, but we're struggling so so much with at what stage do we do it? Does that sound silly?

She's so much quieter since the lump was removed and I think in quite a bit of discomfort due to the return of the lump. This is what seems so hard - she was a lot more her normal self prior to the lump being taken away.
She's eating just fine, but won't drink at all... I've managed to get her to drink by putting little pieces of chicken in some water which she then laps up.
She's definitely not herself. 

The last thing I want is for Tess to be in pain, it's breaking my heart.

I know there's probably not a lot of advice people can give in this situation. But this is our first pet that we've had to deal with this sort of thing with and deep down I'm frightened of the decision we have to make... I think we know what we have to do, but it's breaking my heart..BUT I don't want to see her in pain. :(

Sorry about your dogs osteosarcoma.

My Weimaraner aged 7 was diagnosed with one in her hock, so I know how you feel.

Its a difficult place to operate on.  :(

You are stuck between an rock and a hard place.  Quality of life is paramount and only you know how your dog is coping.

Its a terrible decision to have to make :( x

So sorry  xx

Only you will know when she's ready.  Better a day too early, than a day too late.
Have a lovely day with her doing her favourite things and meeting her favourite people and eating her favourite things before you say goodbye.  I like to add to the happy memories.
I do feel for you.  It's difficult to have that responsibility.  I've been there.  x

So very sorry, this is one of the most difficult decisions that we, as doggy lovers, have to make. I think we have all been in this situation before and my heart goes out to you. You just have to think about what is best for your beloved friend and when there is pain, discomfort or no quality of life, then this is the kindest decision you can make and shows your love for her. She knows that you love her and she trusts you to do this last act of love for her when you feel she is ready.

Hugs