I am sorry for your loss. It is never easy losing a beloved pet, especially so suddenly. Just know that you did your best for her and there wasn't anything else you could do for her. Cats are great at hiding symptoms and often don't show that something is wrong until it has progressed.
Since you say you have a second cat I would take Nignogs to the vet for a blood test to make sure she is FeLV negative and then get her the vaccine so she doesn't contract the virus or can get treatment if she does have it already.
It will get better with time. Just remember all the good times you had together.