First you need to ask yourself "do I actually want to become a game designer"?
Most people think that the game designer comes up with the idea for the game, and such, but this isn't really the case. Ideas are often a collaborative effort by several members of the team mostly coming from different backgrounds. Yes some studios do it differently, but most of the successful studios make games by committee. Also you need to remember that ideas in the game industry are a dime a dozen so if you think you're an "idea man" then you're going to be worthless to 90% of the industry. Back to the topic at hand. Being a game designer means you hash out all the details of a broad design. You map out how the UI is going to interact, or write how you want a system to interact. You could be part of the level design, or mission design team that helps create the world. Also your work is always at the whim of a rogue artist or programmer who "thought it might work better this way". It seems like most people that don't know much about the video game industry want to design, but few are good at it, and most can put their skills to better use in other fields.
While programming is a good/necessary skill to have it always seems like those w/ art backgrounds have a much better idea on how to design things. Most companies can hire a handful of "code monkeys" to do your programming for you, but it's hard to find a good artist, and even then communicating art isn't an easy task. I highly suggest doing art, over programming if you want to move over to design.
Next you'll never get anywhere in the game industry w/out experience. Now it may seem like a catch-22, but if you're persistent opportunities will pop up. Also just because it's not professional experience doesn't mean it's not worth trying out. Companies will hire people w/ modding experience over those w/out, and if the mod experience is with their own software then they might take you over somebody w/ professional experience (don't count on it though).
Last is do you actually want to work in the game industry. Playing games is necessary for the industry, but just because you like playing games doesn't mean you'll like working on them. First off if you're qualified to work in the game industry somebody else will pay you more outside of the game industry. If you make $60k a year expect to make $30k in the game industry. Second you're going to work much harder in the game industry than anywhere else. Though management in the industry is getting better it's still common for people to sleep at work, or pull 80+ hours in a week. Missing a deadline can be very expensive, and you're working w/ some of the most complicated tech projects in the world. Also just about everything you know now will probably become useless in 5 years. The technology and design in this industry change so fast that being ahead of the curve means you're already late by the time your product hits the shelves. So you'll always be learning in this job, and if you take a break from the field you're probably going to need to start all over again.