OK at climbing hills and rarely go into the biggest gear at the back?
Sometimes change gear and wish there was one in between?
Perhaps you should try a narrower cassette.
A lot of road bikes come with fairly forgiving 12-30 or even 12-32, meaning the smallest ring is 12 teeth and the largest is 32. Taking a 10 speed for example, there are some large gaps between some of those gears, meaning you can be pedalling at a certain speed, need something slightly faster or slower, change gear and find yourself way out of what you wanted and struggling to grind the pedals around.
Most cassettes are available with different spreads of ratios in them. A Shimano 105 11 speed is available in 11-28, 11-32 and 12-25. Having your smallest gear as a 12 means you’re effectively limited to pedalling at around 42 MPH (assuming you can’t pedal much faster than 130 RPM), so I’d always start at 11 so you can at least carry on putting some power down when descending, trying to get closer to 50 MPH. (calculate here http://www.bikecalc.com/speed_at_cadence)
So the 11-28 is the narrowest cassette in 105 11 speed which starts at 11 teeth. The 11-32 will probably be too wide to maintain effective cadence.
Changing your cassette is very simple and only requires a couple of tools. If you have any friends who are interested in cycling then you should be able to find someone who already has them.
Read here for a guide on how to change your cassette