is my account and all its posts, comments, activity and bookmarks just gonna be gone?

You will not be able to log into your account. Your account history (profile, posts and comments) will still be visible on other instances (the ones that already exist, any new ones will not be able to fetch content from lemm.ee) in read-only.

Your bookmarks and subscriptions can be exported on the settings page (that, of cause, needs to be done before you loose access to your account). After that they can be imported to a different instance from file.

I also recomend to put a link to your new account in the profile of your old account so that people would be able to find you. It can be done like this: “@username@instance.url”. For me it is “@khorovodoved@lemm.ee”. That way it would work even after lemm.ee will shut down. Only do that beforehand, so it has time to federate.