If I'm correct, you have set a bunch of records like A, MX, SPF etc. in the registry of your domain provider so it would point to the server where the cpanel is running. Even if you change the nameservers to the given one from cloudflare on cpanel, it won't work. Cloudflare works on the root level, meaning that you need to change the nameservers on dns server from your domain provider. Shortly after cloudflare should be confirming it. Pending on you current domain provider you could export all your records and reimport them to cloudflare directly.