What was your motivation? Are you writing because you love writing? Are you writing because you have a burning need to tell stories? Or are you writing because you think it sounds cool and is bound to make you famous and earn you lots of money? If it's the first two, carry on. If it's the last one, then yes, it might be best if you looked for something else to do with your time.I have been wondering was the motivation to write this fiction wrong in the first place?
If the rodent is your main character then you generate sympathy -- or rather empathy -- in the same way as you would do it for any other character, by letting us see his essential humanity, ie his frailty and his doubts as well as his courage and other qualities. If the rodents are simply a plot device for your human characters, then you don't need sympathy for them at all.