Justin, I really empathize with your situation. If there was any way I could be a Vegan again, I would, because the principals are close to my heart. There is a small bit of hope, in the back of my mind, that some day I will figure it out and be able to return. In the mean time, I give thanks for every animal that gives its life for me, so that I may live. I eat only organic, and free range, to put the purest possible in my body, and cause the least amount of pain for the animals.
Anyway, lets see what we can come up with given your parameters. There are basically three ways to get calories into your body. Proteins, Carbs, and Fats. You need to work to increase which ever one is the easiest on your digestive system. If the Fruits are causing you trouble, as they did me, you will probably need to increase your nut intake. Of course these can be very problematic to IBS'rs because of the fat trigger. Experiment with various nut butters to see if you can find one that doesn't trigger your IBS.
When dealing with grains, because you are getting so much fiber from other sources, you might want to try sticking to refined grains. I know that sounds contrary to health recomendations, but the insoluble fiber in whole grains can be irritating. Personally, I eat a lot of Lundberg organic white rice. Try the Sushi Rice, it is very sticky and filling, as well as soothing to your intestines. I also have a gluten intolerance, so I stay strictly with rice, and eat no other grains, but you may not need to do this.
Beans are tough. I don't know what to recomend for you there
I would say that you might want to stick with broken down forms, such as Tempeh, and Miso, but of course you need to be careful about eating too much Soy due to the Estrogenic, effects. Unfortunately, it is not just the amount of protein, but the type of protein that seems to have an effect. Some things to watch for, that indicates that you are needing an animal source, is an excessive hunger. Feeling like you have to stuff your self until you are bloated, and then you still don't feel satisfied. Uncontrollable cravings, and tendancy to overeat. This is something that rarely happens to me any more, in fact I find that I can eat very little food to maintain my weight, because I have balanced my diet. Before, I would eat pounds, and pounds of Fruits, Vegetables, Grains, Nuts, Potatoes (I was a big potato eater) and no matter how much food I stuffed in myself, I continued to get thinner. Anyway, I hope you can find a combination that works for you.
One other approach you might consider, is to try various supplements. Protein powders that are enzymatically digested. If you can handle whey protein, that might be an acceptable compromise to you. I know you wouldn't be Vegan any more, but perhaps dairy might work OK for you. I can't take dairy in any form, but perhaps you can. I don't know about you living circumstances, but if you can, why not buy a Goat? Milk it yourself, take care of it, and love it as a pet That might be a way to get some nourishing animal energy in you that wouldn't be harmful to the animals.
At any rate, I hope you find what works for you. I feel your pain