What are the Sharia implications of my saying, "I have made it unlawful for myself," in response to "May you soon marry a second wife," and does that prevent me from marrying in the future or from returning to my wife?

If by making something unlawful for yourself you meant that you would not marry a second wife while you are still married to your current wife, then nothing is entailed by that, and marriage is not forbidden to you if you desire it. And if you intended to forbid women absolutely for yourself, then nothing is binding upon you either. And if you divorced your wife less than three times, she is not forbidden to you. However, if the divorce occurred before consummation, you cannot take her back except with a new contract. But if it was after consummation, you can take her back before her waiting period ends, unless you divorced her three times, in which case she does not become lawful for you again until she marries another man and he then divorces her.