This spell ... creates a mystic connection between you and the target
It's hard to read this and come to your conclusion that the spell has not been cast on you in some way, shape or form.
Casting a spell "on" someone does not require that they be the target of the spell; if you are rendered unconscious by a Sleep spell then the spall has been cast on you even though a creature is not a valid target for Sleep. Similarly, if you are part of the mystic connection and you are affected by another Warding Bond then the first mystic connection breaks irrespective of if you were the target or the caster.
The spell ends if you drop to 0 hit points or if you and the target become separated by more than 60 feet. It also ends if the spell is cast again on either of the connected creatures.
The caster is one of the "connected creatures."
