Fingers
Well i have to say a a retired runner due to injury my experience is limited but in my humble opinion it sounds like you are pushing yourself to hard evey body/mind has a breaking point and your body will only sustain you for so long before it crashes, it sounds like your body and mind need a time out, relax take 2 weeks off, chill out and rest up mentally and physically then start again in 2 weeks ready for a new challenge, raring to go with plenty of energy and mental strength, there is no point running the NF Mara if your not going to get what u want from it, u know if u dont get 3.15 you'll be dissapointed and then you'll be knackered mentally and physically u wont have got your goal and it may impact your other races which mean more to u, NF was never in the plan just knock it back out, rest up and come back strong for the races u want more, GSR, Windsor etc.
Be kind to your body and it will be kind to you push to hard and you'll get injuries galore - trust me on this i know!
x