I'm very sorry to hear about your situation. I can certainly say that IBS has "ruined" my life, whether it was limiting events I went to (or simply making them less enjoyable), limiting where I would travel to, my activities of daily life, etc. I was 13 when I would have incredible pain for 2-3 hours or so every other week in my stomach. Since then I've been able to eliminate the pain by taking metamucil capsules and colace at every meal with a full glass of water, but the IBS itself remains.
The career path I chose required me to do things that tended to trigger my IBS, things like public speaking, teaching, seeing patients, etc. Although I was on track to become accomplished in my field, keeping IBS at bay simply became too much to stay on that track and I took a job where I would be doing less of the things that trigger my IBS, although you can't avoid stress forever.
Five years later in the same job, I am finding myself moving back to my original track and I would recommend the same for you. The trick is being creative with how you do what you want to do. You mentioned you never had an episode while performing, but it was more the auditions and practices that triggered your symptoms. You can always find a theater company with a more collaborative atmosphere, or perform in a smaller group. I took my profession online and formed an internet business doing what I love to do in a way that no one else has thought to do it. IBS feels like a curse most of the time, so you need to find a silver lining.
If you see IBS as ruining your life, it will. If you see it as presenting a challenge that you can creatively solve, you will approach it more effectively. Believe me, I know this is easier said then done, and almost no one will appreciate and understand how you feel. Its really up to you to pat yourself on the back when you get through little challenges that may seem like nothing to others but take your full strength and willpower to get through.
Regarding treatment, I've tended to stay away from the prescription meds and immodium. The never did much for me but delay the inevitable or make it worse. As I mentioned earlier, I do take metamucil capules and colace with meals which as been helpful in restricting IBS episodes to only those triggered by stress. If you're not on this regimen, you should try it for at least 6-8 weeks and see how you feel. I also carry around GasX which is a bit less disruptive than Imodium but can have a similar calming effect. I haven't taken GasX in quite a while though.
Anxiety about having an IBS attack often causes the attack itself so if you can reduce the anxiety or work through it you're halfway there. You can reduce anxiety by doing the things that make you anxious and getting through them successfully. The more you expose yourself to these situations and successfully stay in them until the anxiety ceases, the less anxiety you'll have next time. Another trick to reducing anxiety is exercise. Getting your heart rate up by exercising most days reduces the level of excess adrenaline in your system, so there is less available to trigger panic when you feel sensations in your stomach. As an additional part of your exercise, try doing situps/crunches and other activities that work your lower abdomen. Not only do strong abs help, but feeling those cramping sensations when you're not having an attack will desensitize you to them over time, and you'll be less likely to panic when feeling stomach sensations. Without the panic and stress, the likelihood of an episode is lower.
Preparation always helps me too. Even if I don't need all of the extra protection or props that I may bring "just in case," having them there reduces my anxiety. Knowing where the bathrooms are also help, but that's probably advice no one with IBS needs.
Finally, some form of relaxation/distraction exercise is good to learn. Deep breathing while counting down helps me. The counting makes me feel that as each second goes by I'm closer to being out of the situation that is triggering this attack. Works well for me on airplanes anyway!
Another technique that is helpful is bowel retraining, which essentially is choosing a time to go to the bathroom for 15 minutes per day. In addition to helping move things along, there is a component to it that could reduce anxiety by pairing bathroom activity with a more neutral setting then the more panic-driven states it sometimes produces.
Hope this helps. There is a very large community that faces similar challenges as you, so you're definitely not alone.