Surgery can be canceled and rescheduled literally at any moment until you're knocked out. That's something I learned here and everyone should remember it.
If you wish you had time for therapy -- you do. NO ONE is saying that if you don't have surgery on Monday you can never have it. On the other hand, if you go through with this surgery on Monday, there is no going back. If you have doubts, anxieties, if you want to speak to a therapist, if you think you need to do more research... TAKE THE TIME. NO ONE is holding a gun to your head. You will not offend your surgeon. People cancel surgical procedures ALL the time. I was on a four week waitlist for my surgery and ended up being taken in within two days because of cancellations. It is NOT uncommon.
It's true that patients with Crohn's limited to the large intestine are considered decent j-pouch candidates. The complications and failure rates are a bit higher than for UC, and overall satisfaction a bit lower, but still considered within the acceptable range. I think with new surgical techniques, the expected outcome of a j-pouch built for a Crohn's patient today *on average* is similar to a j-pouch for a UC patient 20 years ago.
But again, this is all a numbers game. Your surgeon is willing to do the surgery not because she's sure you'll be happy with it, but becuase she thinks it's LIKELY you'll be happy with it. Surgeons can't guarantee positive outcomes, they can only assess risk, and if they only acted in the absence of risk, they would never act at all.
I think you need therapy, I really do, and I think you need it before you undergo a permanent modification to your body. This is actually an issue that comes up a lot with weight loss surgeries as well -- patients think that depression or social anxiety is caused by their obesity, and if they have weight loss surgery and lose weight the mental health issues will be cured. Often those issues DON'T go away, and a whole new set of issues crop up because of the immense disappointment of undergoing a life changing, painful procedure only to be left facing the exact same emotional issues. I would hate to see something similar happen to you.
Good luck.