Acid

Acid stabilizes meringue by lowering pH. Cream of tartar is the most common acid used, but lemon juice and vinegar also stabilize. Too much acid leaves a sour, off taste and should be avoided.

Add acid early on. Whipping might take longer, but the protein network that forms will be flexible and stable against overwhipping, folding, piping, and baking. The meringue will also be whiter.