To me anything with 'self' sounds a little constructed, less idiomatic than other options.
'Excuse oneself' or 'be excused' is possible if you are a subordinate whose attendance is required by someone higher up; it suggests you need to ask permission from the chair, your boss, etc., to be absent. (Similarly, schoolchildren need an excuse to be absent, such as a note from a parent or a doctor.)
Also, the most typical context to excuse yourself is when you're already sitting in the meeting and you need to step outside (to go to the bathroom, take a phone call, etc.), so it could sound a little strange in a different context.
'Dismiss' isn't possible; the chair dismisses the meeting when it's over, but the verb isn't normally used reflexively.
'De-register' isn't really a word, and neither is 'unregister.' If it's something like a conference and you had registered for it but you've now changed your mind, you could ask to 'withdraw (your registration),' but there are other options.
The most common is probably just 'cancel,' as RES-can suggested.
(With regard to the meeting / conference June 8-10, ...)
I'm (so / very) sorry, but ...
I'm afraid ...
(something has come up and ...)
(my plans have changed and ...)
... I'm going to have to cancel.
... I won't be able to attend / come after all.
... I won't be attending / coming after all.
If it's just a one-time or partial absence, you can still say something similar.
I'm afraid I won't be (able to be) at staff meeting this week
(as I have a conflict / doctor's appointment).
I'm afraid I'm going to have to miss the Friday afternoon session
(as I have to catch an early flight).