What's the word for undoing progress?

Solution 1:

The word you are looking for is counterproductive. From the free dictionary:

tending to hinder or act against the achievement of an aim

Solution 2:

Regressing is the word you're looking for, the noun is regression. An alternative is backsliding which is both a gerund and a noun.

Solution 3:

from dictionary.com, REGRESSIVE is appropriate here, in your context:

"This is so regressive"

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Solution 4:

"This is so undermining the group's efforts" (despite best intentions).
"This is so detrimental to the team spirit."

Both words would confer that the efforts erode or diminish past achievements. In fact erode or erosive could be used, too, if the regression is a long-term, unspectacular, rather slow process:

"These measures actually erode public confidence in our policies instead of boosting it."

Solution 5:

If you are "reversing your progress", you retrograde as mentionned by @WillieWheeler.

EDIT after @Barmar comment: If you are "unproductive" (not counterproductive), i.e. stationary (not reversing), you stagnate.

Definition : to stop developing, growing, progressing, or advancing.

Considering your example "This is so... ", demoralizing may fit.