Nope, you aren't being selfish at all. In fact, i think she is being selfish for not considering your feelings. Your health comes first and if she loves you, she should see that. I think you need to have a discussion with her and help her to understand your position. Marriage is a big step, quite possibly one of the biggest steps of your life. It is nothing to leap into at just anytime. It has to be at the right time for both of you.