Right in the moment this order's tall
The narrator is too cowardly to end the relationship himself, though he is unable to nurture the relationship himself. The irony is that he yearns for a healthy relationship, without realizing the amount of selfless efforts that he actually has to put in.
The speaker is aware that what is requested of the relationship will be very hard to fulfill, but the speaker wants this to happen at that very moment.
A tall order is a something that is difficult. Vernon is saying ending the relationship himself is too difficult, or possibly that staying in the relationship is too difficult.
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