You and I are in a very similar boat, and unfortunately I don't have an answer for you about how to keep the weight off when it's hard to eat. I find that one of the most frustrating thing about this chronic disease is how preoccupied I feel with thoughts about my symptoms, the medications I'm taking, what should or shouldn't I be eating, etc.
I also am new to this site. I found it while searching for people who have UC and are having trouble loosing weight. I am just now coming off a 3 month course of prednisone and gained about 20lbs over the course of those 3 months. Finally when I got down below 10mg/day I noticed the insane appetite symptoms start to let up. Before starting prednisone I had been hovering at 210lbs for about 6 months.
I also struggle with the question of what to eat, and actually have an appointment with a nutritionist next month to try and get some advice tailored to my needs. It's really hard for me to keep to a low calorie diet when I am super sensitive to raw food, anything high in fiber, or anything with skins and seeds. That eliminates a huge chunk of food that is low calorie density. The easiest stuff by far to eat (and not have dire consequences) is bread, rice, pasta, and meat... which of course are not prime candidates for weight-loss promoting foods.
My weight loss challenge is compounded by the fatigue that I experience as a result of the UC. Many days I'm so tired that working out requires more energy than I can summon. And leaving the house to go running or for a walk is a challenge because invariably I get to the point in my workout where I am the maximum distance from a bathroom and need to find one immediately.
After great deliberation and weighing of pro's and con's, I'm starting Remicade in two weeks. All of the basic treatments (Asacol, Lialda, Apriso, Canasa, Mesalamine, Cort enema's, prednisone etc) have failed to keep my UC in control. I'm really hoping that with the Remicade plan I might be able to get into a more stable remission state where I will feel well enough to exercise more and eat more fruits in veggies.
I hope your journey is going well! Would love to hear an update of how things have gone for you since February!