It can be incredibly frightening and discouraging to be all set to go out there and give it your best shot to win your ex back, only to discover he's in a new relationship all of a sudden. It can make you lose all hope, and give up completely...but your chance may not necessarily have passed. You may still be able to get him back, and here's why:
Few people jump straight from a breakup into another serious relationship...unless "being in love with someone else" was the reason for the breakup (in which case there was usually something going on behind your back for some time before the Big Fight), being fresh out of a brutal breakup will not be the best place from which to jump into a brand new, head-over-heels relationship. More likely, you're dealing with a rebound...and those are no threat at all, in fact almost GOOD for you!
Rebounds are rarely, if ever, serious...their main purpose is a short-lived good time to recover from a bad time, so that life doesn't look quite so horrible without someone you'd really gotten used to being in your life. A one- or two-week relationship may not even hurt YOU, even if you intend to fight to win your ex back...the confidence and reassurance you can obtain from feeling wanted again can really give you a much-needed boost.
Most rebounds are over and done with in a few weeks, if that...and with things being a little more stable now, it'll be easier to guide things into where you ideally want them to be. Don't fear the rebound relationships, and don't lose your hope if it looks like your ex may be in one.