Perhaps playing on 'nil desperandum' one could come up with 'nil paenitendum'...
Problem is, as QMF says, it has no past participle. I made this one up - following logical assumptions.
WORDS, however, recognizes it... so, if you're not too crazy about being totally faithful to Latin (which I don't advise) you could go with this, pending approval of other members of the Forum.
... or, forsitan, you could use a -tor ending, as in:
Paenitetor, would it be? --> Therefore:
Noli paenitator esse --> Refuse to be someone who regrets.
I don't know about the accuracy of this, -tor endings are not my strength.