Kat - I'm glad you are pushing through this, trying everything you can. Sometimes we have to do this and sometimes we win through, which I very much hope will happen in your case.

A can of tuna is quite a bit of food, unless you mean a mini-can? If you decide to force feed her, use AD from the vet (it's VERY high calorie) with a splash of nutritional supplement. I've found that the best feeding syringe to use is a 35cc rather than the larger ones the vet techs use. Mix the AD up with warm water to the consistency of sloppy oatmeal and make sure it doesn't have any tiny lumps that will block the syringe tip. I keep the mug of feed in a jug of very warm water so that as I draw up more feed it's still nice and warm. Expect protests, but she will feel really good when that stuff is in her stomach.

Did your vet give you Mirtazapine? It's very effective and the dosage is really low, something like a 1/4 tablet every three days. The first day, as you might expect, the cat's appetite is very strong and by the third day it's pretty much back to the way it was before the medication.

Do you know if Sidney is dehydrated at all? That will make a cat lethargic and cause inappetance very quickly. To test for dehydration lift up the cat's skin between the shoulder blades with your thumb and forefinger then let go. If the skin springs back right away, it's a good indication that the cat is not dehydrated. Giving sub-q fluids, if necessary, can make a cat go from lethargic to feeling more normal very quickly.

Good luck! Sydney is lucky to have you as her guardian.