It appears that you can use (non) satis with a result clause to mean 'not x enough to (do y)':
non satis strenue laborabam ut centum epistulas scripserim.
I didn't work hard enough to (manage to) write 100 letters.
I didn't go through every search result, but the only example I managed to find of this construction (as opposed to other places where satis is followed by an ut clause of a different sort) is Seneca the Younger, Epistulae morales ad Lucilium 52.2:
nemo per se satis valet ut emergat; oportet manum aliquis porrigat, aliquis educat.
No one is strong enough to rise to prominence by himself….
Here are some other options, somewhat better attested than the preceding:
Replace tam in your original main clause with adeo non, and then negate the result clause too:
adeo non strenue laborabam ut centum epistolas non scripserim.
To such a degree did I not work hard that I didn't (manage to) write 100 letters.'
Cf. Pliny the Younger, Epistulae 1.8.10:
oculorum porro et aurium uoluptates adeo non egent commendatione, ut non tam incitari debeant oratione quam reprimi; ut uero aliquis libenter educationis taedium laboremque suscipiat, non praemiis modo uerum etiam exquisitis adhortationibus impetrandum est.
To such a degree do pleasures for the eye and ear not require approval, that they should not so much be encouraged as restrained in a speech….
Also, Seneca the Younger, Epistulae morales ad Lucilium 18.1:
December est mensis: cum maxime civitas sudat. ius luxuriae publice datum est; ingenti apparatu sonant omnia, tamquam quicquam inter Saturnalia intersit et dies rerum agendarum; adeo nihil interest ut <non> videatur mihi errasse qui dixit olim mensem Decembrem fuisse, nunc annum.
…To such a degree is there not any difference that he doesn't seem to have been mistaken who said that December was once a month but is now a year.
Use this somewhat uncommon but perfectly sound construction: tantum abest with two result clauses, 'It is so far from being the case that x occurs, that y occurs.'
As explained in Allen & Greenough §571.b, one of the two result clauses (= x in the preceding translation) 'is substantive, the subject of abest; the other [= y] is adverbial, correlative with tantum.' In this case, the latter result clause is negated.
In other words, you could say:
tantum afuit ut strenue laborarem ut centum epistulas non scripserim.
So far was I from working hard, that I didn't (manage to) write 100 letters.
Cf. Livy, Ab urbe condita 44.38.4:
nam tantum abest, ut me hesternae quietis paeniteat, ut servatum a me exercitum eo consilio credam.
For so far am I from regretting my lack of action yesterday, that I believe that I saved the army by my plan.
Replace strenue with some adverb that's opposite in meaning, such as pigre, and then negate the result clause:
tam pigre laborabam ut centum epistulas non scripserim.
I worked with so little energy that I didn't (manage to) write 100 letters.'