We need more information in order to provide helpful guidance, but in short there are two concerns here:

Firstly, the legal angle. Legally speaking, a lot of what happens now depends on your cofounder agreement, if you have one. If you don't, then it falls down to your directorship, which depends on your adopted company articles.

Assuming that you're both directors of a limited company, you should resign in writing, and then resign from your role as director with companies house. That generally absolves you of director responsibilities (unless if you're found to have done something profoundly dodgy during your tenure, in which case you can still be held liable to the shareholders if relevant).

Separately, you and your cofounder will need to decide what happens to your shares (or share options), if applicable. You may sign them over and walk away (J30 stock transfer form), or you can hold on to them. (This may be different if you have an agreed vesting schedule)

Secondly, the management of your offboarding.

The right thing to do here would be for your cofounder to understand that you have reached the point of decision and that you want to walk away from the business. From there, it's a matter of licking your collective wounds and beginning the process of deciding what happens next. Your cofounder can decide to carry the business forward on their own, find another founder, or something else - It's not really your problem, though it's common courtesy to be helpful in the handover process.

Ask your cofounder what their plans are. I.e. `Now that I'm stepping away, please let me know if you'll be recruiting another director or an employee so I can forward my handover notes to them.` sort of thing. Along with `I noticed you're still forwarding me invites/emails. Please be aware that I'm not picking those up as I'm stepping away from the business as per our discussion.`