Make sure your graphics drivers are up to date.
If updating them does not work, check in your osu! folder for a file named osu!.pcname.config.
Make sure values of BloomSoftening and Shaders are set to 0.
Finally, if none of that works, hold shift while starting osu! and enable compatibility mode, or change the value of CompabilityContext to 1 in osu!.pcname.config.