Bees, I'm really struggling here.

My chihuahua/min pin mix has had a bunch of health issues over the past 6 months or so. It started with a weird episode where she lost her balance and was confused and falling down and somehow wounded her eye. Blood tests showed liver problems, but antibiotics brought them back to normal and she seemed to recover. Soon after, the lenses in both her eyes detached, so she had to go on 3 different kinds of drops (for life) to try to prevent glaucoma. But a few months later, one eye developed a deep corneal ulcer and caused a ton of pain and she had to have an emergency eye removal. The vet said her other eye had a tiny bit of vision in her remaining eye and it could remains stable with continued use of the drops. But here we are a month later and her other eye is doing the same thing and we're likely facing another removal, or....it's the or that's so hard for me.

We don't actually know exactly how old Lucy is because she was a rescue I adopted. We know she's at least 13, possibly older. Her quality of life is....well, it's not great, but I don't know if I can say it's terrible, either. She's not in pain, but she can't see, so she's often stressed and confused because she doesn't know where she is/how to get where she wants to go. She wanders around bumping into furniture and walls and scaring herself (which leads to accidents in the house). She basically needs someone with her if she needs to leave her bed...so I get her up to carry her to her water bowl, back to bed, go out, back to bed, eat, back to bed. She is spending most of her time in bed because if she's not in bed she's confused. But like I said, she's not in pain, and she does still get excited about eating...

The other thing is obviously cost. Between September and now we've spent upwards of $3,500 on tests, medications, and surgery. A second eye removal will cost us $2,000+.

She's at least 13 and part of me thinks...will the surgery be worth it to keep her alive for an indeterminate about of time? But I don't want to be selfish and put her down because I don't want to spend the money. But then I wonder about her quality of life. It's not as if I have a clear-cut answer there. Like I said, not really in pain, and it's not like she's been diagnosed with a terminal illness. But then again, all she can do is sleep, she's confused, and I worry about her wandering when we' re not home and hurting herself (she can't be crated because of trauma before she was rescued.) I could go back and forth about this for hours.

Basically, I don't want to end up making the wrong decision for selfish reasons. But I don't know if I can really be clear if that's what I'm doing or not.

How did/ do you make this decision?? Have you ever had to? Did/do you have certain criteria in making the decision?

I'm so lost. Lucy is my first pet and she's been part of my life for 6 years...I hate even having the power to MAKE this decision.