The condition is called Neuropathy. It's not something to be ignored, keep fitting the bike until the issue is resolved. There's no way to exercise nerves. Numbness, tingling, burning and cold are all symptoms of compressed and aggravated nerves and can in time be permanent. No one knows how much time or aggravation can cause permanent damage, everyone is different. Neuropathy in the hands can come from the the cervical spine, thoracic spine or just centered in the wrists and hands. If you can't resolve the issue on your own, get the bike professionally fitted. I'm not a medical professional or a bike fitter, I suffer from permanent Neuropathy and have for more than 20 years.