When you undertake to love someone, or something, there is pain at the end of it - that's the bargain we enter when we give our hearts.
The hardest part of loving a pet is letting them go, but it's the best act of love you can offer, because you release them from the pain that will never get better, and suffering that will only get worse.
If the position was reversed, you can be sure she would do it for you.
Her pain will end, yours will continue, but I know you would rather feel pain now which will lessen with time, than let her feel pain, which will only increase until you release her.
It's time.
My thoughts are with you xx.