I know this is probably a no-brainer but I was over the moon when it worked and I thought I'd share.
I find the character limits on CGPT custom instructions are a too small for my needs. So what I did instead is wrote different custom instructions into a text file and every time I start a new chat, I just drag it over and tell CGPT to override those as the new custom instructions to use for that chat. And it works a charm.
I know it adds an additional task at the beginning of the chat but it works for me. Other handy tip though would be to add a command like '/override' into your existing custom instructions which makes CGPT automatically perform that task. See screenshot.
The caveat warning is just to make sure those custom, custom instructions aren't too long or else CGPT will start making mistakes......that's probably why the original limitations are set.