I second the person who replied, "Ask for help".

Do not use your teeth to carry luggage or other non-eating purposes. Better to let everyone else by. Then to lift one suitcase one step at a time if you can. Better to let other passengers help you.

Ordinarily you should not carry on board luggage you would have difficulty maneuvering. Except there may be unexpected conditions such as the seat you described being upstairs, or the luggage contents may be unsuitable for checking.

Health tips:
http://members.aol.com/ajaynejr/health.htm

By the way there are also rude people who take wheelchairs and who don't need them, just in order to board before other people. This causes problems like no wheelchairs for people who do need them. Thus the need to show proof you need a wheelchair.