If you are continually breaking the same strings in the same place, you probably need to sort your saddles /nut out. Often there will be a very small and very sharp shard of metal in the saddle / nut that your string is rubbing against. You can cure this in two ways:

1) file the bits of the saddle that come into contact with the string

2) get some Graphtech saddles/nut (I Graphtech saddles on most of my axes)