I would move your fermentation vessel to a warmer room. Fifty-eight degrees ambient is going to slow yeast metabolism in a big way. Anything that slows metabolism slows metabolite reduction. Diacetyl is a metabolite. I personally like a touch of diacetyl in British-style beer, but if you are shooting for a relatively ester/diacetyl-free beer, you want the yeast cells to remain in suspension as long as possible.
Have you taken a sample? What did it taste like? Did it have buttery or butterscotch notes? If you cannot detect buttery or butterscotch notes, then you are worrying unnecessarily.
Please bear in mind that you are a new brewer. While wort wants to become beer, one must tackle a rather steep curve when learning to make good beer on a consistent basis. More is learned by making mistakes than is learned by doing everything right. Mistakes teach us the difference between home brewing myth and home brewing reality.