Gerbil question?

I know it's a bit strange to ask about gerbils on a rabbit website, but I'm not sure where else to turn.
I have a wonderfully sweet 4 year old gerbil, Daisy. Over the past few months Daisy's teeth have been splitting. So I've been trimming them every other week for her, and giving her soft foods. But recently, she's just gone down hill. She's skeleton skinny (but still eating and drinking), doesn't move much, and always sleeps. I think she's also now having respiratory issues (She's sneezing and squeaking constantly). She's also developing head tilt. I know she's in pain just by the way she moves around.
Last night I picked her up and discovered she has a huge tumor growing in her stomach. The whole "pear/bell" shaped body, way too skinny body, etc: should have caught my attention sooner. But it didn't. I guess I was in denial or something. I feel so irresponsible!

She's hardly eating at all anymore, not even opening her eyes.

I feel absolutely horrible about her situation and just don't know what to do! I know putting her down would be the most merciful thing. I can't help wanting to be selfish and let her live as long as she can. Daisy has a cage mate, Loki. I also don't know how Loki will deal with Daisy passing. So that's another concern. The thing that upsets me the most, is when I pick her up, she's lively and wants to run around and play. I can't help but think, "Well, she wants to live!"
I just need some advice is anyone has any. I'm clueless and feel like if I put her down, I'll be playing God or something. If that makes any sense. :bigtears:

(If anyone needs any background on my gerbil experience, I've owned many gerbils over the past 10 years. But none of them have died/begun to die like this.)

I would get her to a vet ASAP if you want her to stay around for as long a possible get her on some pain meds or if you think it's time bring her back home so Loki can sniff her and know that she is gone before you burry her, I have done this before with my rats it really helps their grief. I had two of my boys try to warm my old boy up again but when they realised it wasn't working that knew and that's when I took him out. I'm sorry to sound morbid in this post but sometimes we are only keeping them in pain for ourselves.
I hope I have helped I am so sorry that she is sick and I hope everything turns out for the best, it might be something simple that the vet can fix.

Based on your description it sounds like she is suffering and it certainly wouldn't be the wrong decision to humanely euthanize her.

Update:
I didn't have the heart to post an update until now. Daisy began to become more and more obvious with her suffering, so I made the very hard decision to have her put down. I know she's no longer in pain but that doesn't make it any less difficult. It's so hard loosing a pet! No matter how small, they always imprint on your heart.
Rest In Peace, Daisy. I miss you so much my little fluff ball.

So sorry for your loss. RIP little Daisy.