Bronto -
I am 26, and have had issues with IBS for about 4 or 5 years. It is an extremely hard thing to deal with. I definitely thought my life was ruined when I first got it and had NO idea what was going on. I had a bunch of tests and nothing was coming up positive. I've heard people say that IBS is just a diagnosis they give you when they really don't know what else to do for you.
The best thing I can tell you to do, is when you have the diarrhea side of it, just take immodium, and I would tell you to be careful what you eat, but I do know that anything you eat will make you go. It is terrible though, because it really does dictate your life in many ways. For instance, when I go to the movies, I have to sit near the exit so I can have easier access to the bathroom. And I have to coordinate my meals to what time I have plans, so I can try and prevent it from happening. All of these things are inconveniences for sure, but unfortunately, it's what you have to do to cope with it.
Stress is definitely another indicator of how your IBS will flare up. It's obvious you cannot predict any kind of stressful circumstance that occurs in your life, but again, it's about coping. If you do what you can to minimize stress levels, your IBS should remain at bay. I was put on Zoloft because of stress that ultimately caused my IBS, and it has helped. In my opinion, you are too young to be put on an antidepressant; that is unless there are valid reasons, such as extreme anxiety or obsessive compulsive tendancies.
Keep finding people that can support you and empathize with you, it is the best thing you can do, trust me! :)