HUGS!
Never good news when your piggy has been diagnosed with an incurable issue and never easy when it is a problem where you have to decide when the time has come. It is emotionally a very difficult situation as your grieving process has start right now.
Please make sure that you put your piggy's wellbeing/pain before any desires of your own and that you reassess constantly. We have at least this last, most heart-breaking but also most loving and kind gift to give.
You need to evaluate just how much support you want to give in terms of syringe feeding as the discomfort of the mass is boviously already affecting her because it is very easy to get caught in the trap of supporting longer than in your piggy's interest. You can support feed her from a spoon instead of a syringe, so it is your piggy who controls the process and not you.
I have found this very helpful with borderline piggies of mine in comparable situations to help me balance quality and zest for life with a life-threatening problem and to allow me to make the decision when to pts/euthanise on a clearer basis and with less in the way of the inevitable doubts and feelings of guilt that come afterwards and that are an integral part of the grieving process.
Complete Syringe Feeding Guide
Your girl is a grand old age; take pride in that! Please concentrate on making the best of what time you have left with your precious girl to say and do whatever needs saying and doing. Love transcends time; you can pack a lifetime's worth into just a mere moment. This is the one great gift that comes with a terminal diagnosis; it is a very bitter-sweet gift, but please do not squander it as it is going to make dealing with your bereavement afterwards a lot easier on you if you are only left with the pain of the loss and the sadess but not any unsolved conflicts and anger.
Sadly it is an experience of mine that the more you minimise the risk of any avoidable illness/condition with your good care the more likely your old piggies are coming up against something nasty when the body is going wrong or an illness hits with lightning speed due to a no longer flexible and fully working immune system. Internal masses seem to be one of the more regular problems, as are major organs like kidneys, liver or the heart going wrong.
None of us can choose what our pets or our human beloved ones die from, but gently drifting away in your sleep is very much the exception and not rule, unfortunately, even though it has become our very sanitised nebulous concept in a society where death as been very much pushed to the side and as a result is leaving us without any coping tools and mechanisms.
PS: If it is any consolation for you, I am currently in a similar situation with my 7 year old Hafren and have been assessing on a daily basis whether she still has the will and the quality of life since the new year while on palliative care. You have my sympathy!