I’m being separated under QMP. My adjusted ETS date puts me 55 days short of reaching 18 years of service. Any advice on how to fight this?

COL Mikel J. Burroughs Sounds like you could use some advice.

SFC (Join to see) Business rules for separation under QMP are perishable, as the rules are updated and published as part of the QMP board announcement.
That means I could only guess at an answer and and can not even do that as your post lacks key points to make even an educated guess.
Gather your supporting documentation, AR chap covering separation under QMP. QMP procedures, QMP announcement (which i think is MILPER Message 17-308) and go see post legal services. They are there for just this kind of thing. Remember they can not tell you what to do (make a decision for you) but they can help you understand your choices.

All that said....the Qualitative Management Program is as a whole a good tool for the service.. It is sometimes harsh to the individual, some times marginally "unfair" but always black and white. either selection for separation is warranted under the business rules for this years board, or not. If selected, you either met criteria to be separated or did not,,If not, THAT is your fight...
If you were selected properly by the rules of the board set forth....IMHO even if it is harsh, it should not have been a surprise. Not is it something I would support a Soldier fighting for unless there were extenuating circumstances ....and there can be.....but they are not common. out of more then a few that asked,,,I assisted only 3 QMP selected SM in asking for exception..all three granted for that board...and what I told them, this was not a pardon, just a stay of execution.. Get your things in order, make a plan...its going to happen next time.

SFC look up AR 635-200 Chp 19 para 15. For everyone on the board, does this not apply??

b. Soldiers with a minimum of 17 years, 9 months of active Federal service at the time of notification of QMP selection,
who choose not to appeal, will be retained to 20–year retirement eligibility upon request.
(1) Those who appeal will also be retained to retirement eligibility upon request if the appeal is denied. Soldiers in this
category must apply for retirement (see chap 12) to be effective no later than the first day of the month following the month
in which they complete 20 years of active Federal service.