My condolences on your loss, I am sorry. Personally, I would explore things with the vet before making a decision. Diabetes can be managed, although it may take a while to get the dosage sorted. If you feel you can do the injections, then why not. Willow will have adapted to poorer vision and hearing, dogs are way more adaptable than we are. Regarding the ongoing cost, it may be worth looking to see if you can buy the medication online. Your vet will need to give you a prescription, and that will be chargeable, maybe £20ish. Regarding her weight, the days are longer, the weather is bound to improve, I hope. If Willow feels a bit better with medication, then maybe more walks? It would get you both out, and that can only be a good thing. Keep in touch and let us know how you get on. Best wishes.