Yay! You now have the leverage to get someone else / more resources onto this project with you!!!
Prior to the email, you felt overwhelmed, but going to anyone for support you had no leverage. Since mistakes are being made enough to warrant the CEO's concern... that letter is your leverage. It is now patently obvious that this particular project needs a team, and not a person flying solo on it.
You turn this letter into a "Thank goodness!" & smile & ego stroking opportunity (for your boss... the whole... I completely agree with you, the mistakes are far too critical, and I'm so glad that we agree, etc. so forth and so on,) and use this 'might get fired' into a kudos. And a leadership position.
My suggestion would be to write up a proposal showing both the problems the former employee was having, and that you are having. The areas of weakness and level of time needed, and you problem solve the heck out of it.
And now you've got the tacit approval of the CEO to back you up. Instead of making it a personal problem WITH you (overwhelmed, can't handle it, etc.), you make it a problem to be solved BY you (with that proposal that outlines very clearly the problems with the project and how they can be solved on your end with the additional resources).
Just my .02