I guess it depends what you want to go back to school for, and how long you are willing to be in debt. I am still paying for my undergraduate degree 8 years after graduation. To me it was worth it, but it was for a undergrad degree. I am about to go back for my Masters, part time though as I need to continue working FT.
Obviously you already are a doctor of veterinary medicine though. Can I ask what you want to study? Why are you not using your vet education, or do you need to validate it in Canada (and honest question, I don't know if you need to do a competency exam or something before being licensed here).
I'm assuming that you are divorced, or getting divorced, but I don't know your history and how it applies to your situation. Do you have kids? Are they with you, are you paying support etc? I think these questions are relevant to what decision you make, especially if there are kids involved. Going back to school full time (which I am assuming you will be as you can rarely get OSAP if you are working) is a financial burden and a big stress, and you don't want your family to suffer from your decision.
Guelph definitely is the best vet school in Canada, but there are only a few and it is the only one in Ontario. If you are planning to go for this it is likely your only option, unless you want to travel out of province or internationally.
Good luck!