Is saying, "I wish this were in me and not in you," considered a supplication against oneself, and can God answer it despite the supplicant not intending to transgress against themselves?

You erred in your supplication mentioned and went against the Sunnah. For the Sunnah is that the visitor supplicates for the patient for healing, not that they supplicate for illness upon themselves in their place. We hope that your regret and seeking forgiveness are sufficient. Accustom your tongue and increase in supplication for well-being.