Re: I'm scheduled for gastric bypass surgery...
Realize that each stage is different. There will be a few weeks of very difficult. Then you will ease into the honeymoon window where it's very good. You are able to eat small meals, be satisfied and the weight will fall off. After a couple of years it will get easier to eat more and you'll have to go back to a "watch it" mentality but at that point you will be much thinner and have far more options in terms of activity to help offset the calories. But remember that no stage is permanent. A lot of people say the first couple of weeks they get very depressed in that "what have I done to myself" phase. But that will pass and you will do well.
Good luck. It's a hard decision but one you will probably be glad you made.