Half familiar, breathless, restless longing, half exasperated reluctance to spend another day in turmoil; if this message arrives prior to an arranged meeting, a clear addendum is attached - I can't wait to meet you - and thus conveys excitement, joy and exuberant anticipation of the encounter.
Should this be the case,
Squee,
Likewise or
Obsession may all be suitable replies. However, if given in response to a more delicate message – perhaps
Don't give up,
We can get through this or
Anything but that – it slants dramatically towards I won't wait, and takes on a far harsher tone. If so,
Anything but that appeals the decision.
Consequences warns of potential trouble as a result,
One last chance asserts the limits of the Receiver’s tolerance in militant terms, and
Patience promises great reward should the Bearer choose patience, whereas
Is this the end? expresses concern over the possible consequences of their haste.