Offsets 10, 8, 25, 6, 6, 14, 21, 11, 12, 13, 11, 12, 15, 21, 1, 7 (that's a key length of 16) yield the following decrypted plaintext: "failurearrivedquicklyatgjmshandsastheirprogramdefeatedmedepressinglyquicklyimustconcedethepointiwasoverconfidentandunderinformedandgjmfreakingwins".

The actual process involved manual intervention, though I think a smarter program could have done it automatically. It went like this:

• Consider key lengths up to 20.
• Instead of picking the single "best" shift at each offset within the key, consider all shifts that come within 1 unit of best. (Meaning a factor of e in estimated probability.)
• Display all the decryptions resulting from these options, unless there are more than 300 in which case display 50 random ones.

This produced a big long list of candidates. None of them was right, but some of them looked more credible than others. Keylength 16 seemed to let us do the best, so I focused on that and picked one of the better-looking candidates, which looked like this:

failhrecorkveefu
ickllatigmuhaoss
asthrirrooiranse
featrdmgaerrethi
nglyduiehlaimvht
concrdeveeroioii
wasoierelnhidfct
anduadetfnhorntd
andgwmftbaminhli
ns

which has lots of plausible bits in it. It looks like it should maybe start with "failure", a plausible enough word in this context, so I tweaked the fifth offset (yes! lots of other things got better too), and continued tweaking one by one as likely words and word-fragments jumped out at me. After a few tweaks I had the decryption above.

A program clever enough to do this by itself would need to have some inkling about digraphs or trigraphs or something, and then it would need some kind of iterative search procedure rather than just picking best offsets independently for each letter. Not terribly difficult, but it seemed easier to do something simpler-minded.

[Edited to add: I think the sequence of giveaway words for the tweaks was: "failure", "quickly" #2, "overconfident", done.]