I have only run 2 marathons ... Columbus in 2006 in 3:26 ... qualified for BOSTON ... managed to save the money ... and train all winter on snow, ice and hills! ... planned to do similar to Columbus in Boston since I'd trained so hard I figured that better training would equal similar time on harder Boston course ...
what happened? Boston kicked my butt and I finished a full minute per mile slower than I had in Columbus the fall before ...
was I bummed? ... at first, NO - I was thrilled I actually managed to finish the thing! ... when I got back to the YMCA I work at and everybody already knew my finish time, many had followed me on the internet, and they were very happy for me ... cool beans! I should have been elated ... but I felt similar to you ... a bit disappointed and fighting the urge to give my long list of excuses even though I was the only one not thrilled with my accomplishment.
Now ... a year later ... I am beginning my training for another hopeful qualifier in the fall and hoping to run in Boston again next spring and give it another go ... better mindset, better weather, and at least as good training should help this time ...
I'm proud I made it to Boston; and FINISHED .... out of the running community, (let alone the entire population), there really isn't that high a percentage that can say that ... well ... there is YOU and "some" others here
Be proud and don't dwell on what coulda/shoulda/woulda been ...