Could I be a writer if I keep working on it?

Those are all careers that are hard to make a living at and way too competitive. Back up plans are good but that's a bit like saying you want to be a rockstar and bank on a record deal and having a back up plan as being a movie star. Writing is good but it looks like your plan is to write novels. Again, that's a very cut throat world and hard to get into. For everyone that gets published and has a hit novel there are 1,000 more that never make it. Your best bet is to at least minor in something like business or marketing. Entertainment isn't a business that requires a degree. You are either really good at what you do and get really lucky or you do not. A theatre major isn't going to get you a good job if things don't work out. You really ought to consider something solid as a back up plan.