Whilst I understand and appreciate your reticence to ask for help/ tell others about your condition a simple call to You First just to mention your worries / concerns would i'm sure find it's way onto a briefing sheet for the CC for the flight and whilst technically they aren't supposed to put bags in overheads i'm sure CC would look to assist you if they could and it didn't fit for some reason in the cupboard/ locker and they would be just know to be generally more mindful towards you. Hope the flight goes well