It sounds perfect: You own a mare—especially a mare whom you've enjoyed in competition or in years of trail riding—and you think you should breed her before she becomes too old. "Wouldn't it be neat to have another horse just like her?" you dream.
But it's nowhere near that simple. Breeding your mare is going to cost you real money, possibly a ton of money—and perhaps much more emotionally—just to get a live foal who can stand properly. And breeding isn't cloning—while the foal may be similar to your mare, he or she won't be an exact copy.
Breeding your mare and producing one or more foals is a commitment similar to raising your kids. And while a foal shouldn't be as expensive as your kids—because you won't have to send him to college, and he won't get arrested—he's not going to be cheap. Raising a horse from conception to maturity is a commitment of expertise, time and money, over a period of years.
That's why you need to seriously evaluate yourself before you evaluate your mare and consider her potential mates. How do you think your life will likely evolve over the next four or five years? Are you planning to have your own children? Are you going to go back to school to further your career? Do you think you might move or become more deeply involved in your career? Or is your job—and, thus, your finances—in jeopardy? Do you have a sick relative who'll need your care? What will happen if they die?
Obviously we can't foresee how our lives might change unexpectedly, but if you think a major life change might be coming in the next few years, this is probably not the time to breed your beloved mare.