Hello!
I have also had a problem with watching my breath, because of my childhood asthma (which started very early, before I learned to think rationally). Luckily, watching your breath is just one of many, many techniques to bring about meditation. You can also count silently to 4 (as I did) or to 10 (as I have seen recommended elsewhere). Or you can focus on a mantra, as taught in the free meditation course here at this website. (In fact, you should see a link to it on this very page.) What is important to watch is not really your breath but your mind. You - the part of you that observes all things - should keep track of where your mind is. This is why you bind it to your breath, or to some very simple mental process: So you can keep track of your mind, and it doesn't run all over the place. The breathing is not important.
Incidentally, you will start breathing again automatically after a while. If your strength of will is great, you may be able to keep your breath until you lose consciousness, at which point your body will breathe on its own. To end your life by not breathing, you need to be a master yogi and have reached the end of your natural lifespan, in which case you may go into Mahasamadhi and exit your body voluntarily. You and me are probably both a bit away from that.
