is it time to put her down?

Ive had my kitty all my life. I'm 18 now, and she only.barely turned 12. She has been in great health, except about five or six days ago she had trouble keeping food down, along with a fever. She hasnt eaten regularly since then, though just last night she eagerly gobbled up solid cat treats which she wouldnt even touch a few days ago. The vet has found a small mass in her lungs that they say is removable, but theyre not sure what it is. And the past three days she has not been.able to walk normally; she tries vigorously, but one of her hind legs is bothering and her and has been leaving her mostly laying down and frequently shifting uncomfortably. She also was diagnosed with cat asthma just three days ago, and has been seemingly having trouble breathing comfortably since then. She has periods where she breathes perfectly, but then other periods where it seems harsh, even though her breathing is inaudible. I love her so, so much, but there is nothing that i can do to help her. I was planning on getting the lump removed, but the vet says he doesnt want to do that until he understands the cause of her other problems. I dont want to keep her alive if she is just going to suffer and be confused at why she's constantly going in and out of.the vet.... i just want to do what's right for her. She still seems very alert and always purrs and acts happy when i interact with her. But do i wait until all of her enjoyment of life is gone to.put her down or do I do it at a time where she is not completely miserable? Any advice or experience would be so appreciated... this is the hardest thing I have ever had to do, but she has always been such a good, sweet girl, and i don't want to fail her now or cause her pain.

Has the vet said anything about her heart?  Some of what you describe would go hand in hand with heart problems as well.

If your vet thinks that there is time to think this through, then I would give her more time.  Ultimately, it's your decision, but I'm a firm believer in the idea that cats will let you know when they're ready.  She is enjoying your company and is still bright and happy.  Give her some time.

I agree, but I also am wondering what the vet thinks about the mass. 

Asthma is easily controlled, not a big deal.

It seems to me that the sooner that mass is removed, the better chance of her making a recovery here.

12 is not old. 

What is the Vet saying about whether or not she is suffering, her long term health, etc?  Are they advising you one way or another?   Maybe this quality of life chart can help:  http://www.veterinarypracticenews.com/November-2009/Quality-Of-Life-Scale/

I'm a little confused as to how they can say they can remove the mass in her lungs if they don't know what it is , but I can certainly see how having a mass there could cause some issues breathing. 

Definitely discuss with the Vet about her heart (via phone if possible...make sure they checked it), because that back leg issue could be from a blood clot (although normally it's BOTH legs that are affected).  Is that leg cold to the touch?   Also, is she's coughing, which I suspect since she got an asthma diagnosis, that also can be a sign of heart issues.