Interval training and ride more. Not knowing how much you ride now I suspect it's not frequent enough. A new bike my ignite a new flame and get you out more but may not increase speed unless something is seriously wrong with current bike. I like to chose places in a ride where I will really push myself hard and follow it with a moderate pace to rest up. I will do this a lot during a ride. A year and a half ago I averaged 14mph and now can be as high as 20 but usually 18. Intervals work. Give it a try.