Sorry, that is obviously bad UI/UX design on my part. The button on the left is actually a Continue-button and the second button a Play-from-start-button.
There isn't a way to restore a previously overriden save (remember this is a jam game, so I didn't put all too much thought into the save system), but there is a way to skip levels by manipulation the save data, at least if you’re playing on Windows. It's stored in the registry though, so it should only be manipulated by users who are familiar with registry editing.
If you are, in the registry under HKCU\Software there should be an entry with my name ("Fabian Denter") and underneath that an entry for the game. Then there are some values editable, one of which is "Scene to load" which you can set to a specific number. When you go back to the game and press the leftmost button in the main menu, it should load the level with the index you entered in the registry.
But please ONLY change numbers in the registry when you are sure to edit the right ones!
I sincerely am sorry for this and hope you are able to go back to the point in the game you already reached!
Edit: I looked up where Mac saves the game data and it should be stored in the ~/Library/Preferences folder in a .plist file. I can't check this though, because I don't own a mac.