Sorry for the delay in my response guys, but I've just returned from Captain Lou's Marketers Cruise...amazing.
If you feel like you are being caught in a rut with short copy and it converts, I wouldn't consider that a problem

. Depending on what offer you are running, "Exposed" might be what you are looking for.]
However, a lot of the time redundant themes can crop up, and the best way to deal with those is just to explore your "writing theme" in depth.
For example, if this were an offer letter, it might have originally started off as "High Converting Short Form Sales Letters Secrets Finally Exposed!", but what makes this theme compelling?
Most importantly, why do people care about Short Form Sales Letters?
Now this is something you don't have to expose, this is something that can be discovered or revealed!
"Discover How Short Form Sales Letters Can Almost Magically Turn Any Offer Into A Cash Spewing Machine!"
Or even highlight an advantage...and especially highlight the strongest benefit.
"STOP Wasting Time With Long Sales Copy That Doesn't Convert! Discover How Short Form Copy Can Quickly Optimize The Earning Potential Of ANY Offer!
As for "having enough" to write a long-form letter, just try and remember that a sales letter is as long as it needs to be. Repetition is important, as well as writing about every single benefit you can think of. Stories are excellent ways of relating to a reader the experience of using a product. All of this will contribute to the overall length of the long form....but ultimately it doesn't matter "long" a sales letter is, what matters is the reason you choose to make it long or short in the first place.
Hope this helps!