Mashed in at 190? F. Next time go even higher, like 200? F. Or insulate mash tun. Or both.
Based on boiling 12.00 gallonsliters at 1.046 for 60 minutes, this will decrease your wort by 1.00 gallonsliters, bringing your after boil OG to 1.050. You wanted an OG of 1.051. It looks like it will be under.