I found a misspelled word after approving print proof for distribution, do I have to retire the project and start over. The spelling error is in a chapter title.
Correct the mistake in your computer file. Upload the PDF of the corrected file, and make a revision. You'll have to buy another proof. That's all.
Alternatively, let it ride. One has to be realistic in one's expectations.
If you think you are going to sell 10 1000 copies then it would probably
be for the best to correct this error. However, if your experience will
be that of the majority of self-published authors you may just want
to say 'meh'.
Don't leave it uncorrected. If you do that, you'll worry and lose sleep. Your health is more important than the few dollars or euros a new proof will cost you.
Indeed. It's bad enough not spotting such things to correct, but to not correct them when spotted is not good practice.