Hi @Jailbait, let me see if I can help out here.
Have you tried rebooting your phone in safe mode to see if the phone detects the headset there? Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated, that way you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
If the phone still does not detect the headset while in safe mode, I would suggest that you try to repair the software on the phone using Xperia Companion and a computer. Keep in mind though that all user data will be erased by the repair, so it is advised to do a backup before to avoid data loss.
If none of these measures manages to improve the functionality of the headphone jack, I believe that the best thing to do is to get in contact with your local support team in order to have the phone checked by an authorized service center. If you have any questions regarding the service, such as service routines, warranty and consumer laws, these kind of questions are best answered by your local support team as well.
Try this and feel free to keep me posted with the results!
