Hi Shelby,
A certain amount of codependency is good, maybe even necessary, for any functional relationship. However, too much of it can be harmful, and I can see some signs of it when you say you have to be on anti-depressants or smoke too much in order to be what someone else wants you to be.
It is perfectly normal to feel responsible for another person's happiness in a relationship, and to give in to their needs on some things, but if it leads to erasing your own personality, it is not healthy.
From relatively small things like where to eat dinner or where to vacation, to relatively important things like not being able to dress and behave the way you want, despite your partner knowing your situation, it seems that you are sacrificing too much of yourself at every step. I am not sure if you and your girlfriend have thought about the future of your relationship, but if marriage and/or a family is something either/both of you are planning, have you thought about whether you will want to be in the role of a husband and a father if you are already having trouble fitting into the role of a boyfriend, and that too a restricted definition of a boyfriend?
You have every right to make an individual decision about your relationship, and fear of someone's negative reaction is not a good reason to put it off. Maybe you can get someone else to help / intervene if you fear for your personal safety, but other than that, if you have to let go of someone / something to be happy, it is better to do it sooner rather than later.
Hope that helps.
Take care
Cindy