I had this with my mountain bike. A colleague who is a very keen cyclist (ie. cycles 100 miles before lunch at the weekend etc!) recommended I heighten the handlebars so I’m sitting more upright.

He said it was due to core strength (or my lack of it!). He said another way to improve it was to sit upright on the bike with tiptoes on the ground then with your core muscles engaged, gently lean forwards to rest your hands onto the handlebars, but not to load them with the weight of your upper body but to use your core muscles to support your upper body.

Sorry this probably doesn’t make too much sense but when I tried it, it did make a big difference!