Er... go slower? ;-)
I'm about the same speed as you for the shorter stuff - 49 & 1:20 - but 1:47 for the half. And 8 min miling early on is too quick - only by 10-15 sec, but too quick anyway. By the time you did a slow mile 7, the damage was already done - you'll still be paying off mile 1 at mile 13.
You're absolutely right - you should be capable of 1:50, but you have to throttle back really early on, probably earlier than you suspect.
Of course, if you're not doing enough miles in training (a long run of 13-16 miles, and maybe 25-30+ miles total), or not hydrating properly, you may suffer anyway.