I've found that often I will attempt to beat someone down and as usual just bring their shields down on the first strike. But then they one hit me back and I die just like that ( full shields and health )
while you tried to beat them down, they prob kept shooting, so when your beat down animation was over, they hit you. with the extra shots they put into you they'd have the advantage when beating you down.
The beatdown is by FAR the most powerful weapon in Reach. Plus the fact that you can melee fairly fast.
As for your problem, maybe there is a variation of a "BXB" or something. Other than that, try jumping back right after you press the melee button. It creates a little space and can save you. Then pull out your pistol and you'll most likely be aimed on there head by default. Then boom, headshot biatch?