Besides being a tad too small for your 5'9", just by looking at the seat's height, makes me suspect you're riding with the seat too low. The frame looks like a 52cm, possibly.
Are you straddling the saddle when you place you feet on the ground? If so, that means your riding position is wrong. You are roughly the same height as me. And I see the use of the extended stem to compensate for the frame. The seat post should be higher if I were to be fitted on that frame. 4-5cm longer top tube means it should be roomy enough to be ridden. Rule of thumb, adjust the seat height until you are flat footed and leg straight. Set the seat's fore/aft position by having your knee vertically above your pedal when the right crank is at the 3 o'clock position.
Not sure of a Brooks, but the saddle should be horizontal to 1 degree down from horizontal.