I used to do that too! Now that I have long nails, though, it feels weird when I have to file them down. The simple answer is stop biting them. However, that can be difficult, especially when you bite them out of habit.
If you're having trouble with that, you could wear a clear nail varnish. Every time you put your hand up to your mouth to bite your nails, the varnish will remind you not to. You can also buy varnishes which are specifically designed to care for your nails.
I have naturally strong nails, but I find that they become more brittle in the winter, and are therefore more likely to break, so if you're trying to grow your nails, try doing it over the summer!
