Why are people suggesting you to destroy the hard drives? Assuming you will donate the computer, it will require the hard drives with it to be usable.
Check this tool. You will need a low level format. Problem solved without damaging anything. Hard drives will be usable and data on them will be unrecoverable.
I could have access to my wife’s FB and she can see mine, we do that all the time without hiding messages from eachother.
BUT
To demand that, to want that from you in the first place, even if you are ok with that, its a major red flag. Yes its possible to make him login without accessing your email. Those are different passwords anyway (they may be the same, thats on you, but they are based on different platforms and hosts). But to make him do that is to bend to his will.
Today, is your facebook account. Tomorrow, your instagram. Then, the email. See the issue?
My advice (and many other users here): tell him goodbye. I know it may hurt, but its the best for you on the long run.