I would say have the surgery but have the vet agree to call you once he’s started the surgery and if he thinks it will go well the carry on. If he looks at her injured leg and thinks it could be a long drawn out recovery then don’t wake her up from the anaesthetic. you will have tried at least, you wouldn’t want her to go through more pain and more surgery.   

This way you gave her a chance but couldn’t go on  without causing her more suffering. 

I don’t think you are heartless at all.