Don't feel bad, it's not your fault and they should be glad that you are still willing to come to work whenever you can. I have employers that treat me badly for being sick as well, and even have a boss that refuses to believe that I have a real illness regardless of how many doctor's notes I give him or internet articles or brochures I hand him. I know how being looked down upon for missing work feels despite how hard you try not to be gone, and how being treated poorly for being sick feels, it's really horrible and I feel bad for you.
What has made me feel better was that yesterday I talked to my very smart and caring friend. He said it sounds like the problem is with my boss, since a couple other employees here have crohn's too and nobody treats them like that. He wondered if I spoke to the boss's boss about it and I said yes, and that they are being a bit more accommodating with me missing work from being sick or for doctor's visits but they are a bit hostile about it, and that when I mentioned that it seems like the problems are related to this one boss guy not believing me, his boss's reaction was to say "well hopefully we won't have to force you to go on disability". I felt like that was a threat, i.e. "hopefully we won't have to fire you" but my friend said "Disability? That's great!". He explained to me that disability is 60% of your income, tax-free, so basically like getting paid to be at home. He also said that whenever your employer treats you differently in a negative way due to a medical ailment that you can sue them and either win a big compensatory judgement or just have them settle out of court quickly with you for a decent amount of money. He said it sounds like I have the potential to be very rich due to mistreatment. I would say the same goes for you, since disability/equal opportunity is a federal law. Let them treat you like dirt, you can hit them in the pocketbook for it later, and they can't fire you, they can only force you out on disability and you pretty much get the same salary you make now since it's 60% but no taxes.
Hope that cheers you up, I know it's not exciting to look forward to disability pay or to put up with poor treatment and get a monetary settlement in return, but it's good to know you won't be in a financially bad situation if things don't go well with your job. Concentrate on feeling well with your health and finding things that keep you happy! I know it sounds weird but I buy little stuffed animals or figurines or look at kitten pictures on the internet when I am sad, even little things like that can turn around a bad day and help you forget a little about feeling sick 
Speaking of kittens, if you like them http://kittenwar.com is thousands of adorable cat and kitten pictures!