Play around with the seat and bar position. If you have too much weight tipped forward onto your hands then numbness can set in. Trying lifting and/or tilting the bars slightly or even tilting/sliding back the seat position slightly. You want a natural position where hands are resting on the bars, not supporting your weight.
Also, drop your shoulders. All too often we catch ourselves with straight arms and locked elbows. Drop your shoulders, bend your elbows and use your core/abdominal muscles to support more of your weight. The difference will be quickly recognisable.