Trying to insert and not type-over

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When I double-click a cell to add text to a cell, I can add additional text after the entry, but I cannot insert text in the middle of an entry, as it will overwrite any text thereafter. It just started happening yesterday. I think I was typing to fast and pressed a button(s) that activated this function and now I have no idea how to fix it. Please help! This is driving me crazy lol.

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