Make sure to eat regularly and drink loads of water. Coming from outside the country and having IBS, you may want to consider sticking to bottled water. Other drinks are okay, but plain water is going to be best for staying hydrated. If you need to, set a timer for food (I forget to eat, especially when I get hot or anxious). Try to get meals at off hours when there are fewer people trying to get their food. If there are snacks you know are safe that are transportable, bring some with you so that you know you have something you can eat pretty much at any time. As much as possible, mostly keep to a similar diet as home. I know there are loads of yummy foods at WDW that you can't get at home, but you don't know how your body will react.
How much control will you have on what you do and when? If you'll be able to have input on what park you go to each day, I'd suggest looking at some of the tour planning websites that have information on what crowd levels will be like in each park. If someone in your party will have a smartphone with a US SIM (or is willing to cope with roaming data charges), I'd suggest having an app that will give you live information on wait times for attractions. If your dates haven't been booked already, I'd check the websites with crowd information even before booking. American universities give students a week of holiday in March. While they aren't the same exact week, there are 2-3 weeks in March where a large number of unis are off during the same week and WDW can be much busier during those weeks.
Regardless of how crowded the parks are while you're there, try to take a break - preferably during the afternoon when it's usually hottest and sunniest. If you're staying at a WDW resort, go back to your resort and swim or take a nap. If you're staying offsite and can't go back to where you're staying, find a quieter area (in one of the stickies there's a link to a guide for guests with cognitive disabilities which is also useful for anyone looking for a list of quieter spots) or some of the less busy indoor shows. You can, if needed, lie down in First Aid for a while, as well, to get away from the crowds. If you're staying on site and depending on WDW transportation, try to use it at off times. That can be difficult since times that are best for avoiding crowds in the parks are not necessarily ones that avoid crowded buses (i.e. rope drop).
In the US, there generally aren't separate "disabled toilets" the way there are in many European countries (I'm only familiar with the US and Europe). Most wheelchair accessible stalls are just a larger stall with bars in the normal restrooms. There are companion restrooms at WDW (mostly aimed at use by disabled people who need assistance with toileting/using a restroom), but there aren't many and can have longer waits than any regular restroom will. I actually think, for you, that using the companion restrooms would likely open yourself up to more chances of a huge anxiety trigger than figuring out how to manage with the normal restrooms (you might want to look at the recent thread about bathroom etiquette to understand why). If possible, you may want to consider trying to use the general toilets with multiple stalls more often at home as you get closer to your trip.
While you're at WDW, use the restroom as much as possible. Definitely use it before entering any attraction queues. I have a "see a restroom, use it" policy at WDW. While it won't eliminate all chances of problems, it will substantially reduce the chance of being in a queue or on a ride when suddenly you need a bathroom immediately.