I would create templates (canned messages) and work on them now, before your emotions get the better of you. Thank somebody for the invite, leave space to customize the invite to say specifically why you appreciate it, provide several interchangeable options for yourself to mean "darn it, it conflicts with an existing commitment for me!", say that you would appreciate an invite in the future, AND that you hope to be able to reciprocate the courtesy by inviting them yourself when you are hosting.

Spend some time with the templates.

then, when you need to fire one, just customize and fire.