"Sum, sentio, cognosco", or something along those lines? The idea here is that we exist, and we work from that basis. No ergo involved. Maybe add "I act" to the end: sum, sentio, cognosco, ago.
Yes I an thinking too possibly Godfrey that Epicurus might not like the "ergo." But I presume at times (lots of them) Epicurus did use the equivalent of "ergo" so the issue is more subtle than I can process right now.
Non fui, fui, non sum, non curo
Good point that we know that one. So I definitely think "sum" is correct and the issue is what short Latin makes sense to precede it as evidence, plus what "connector" makes sense ( ergo?)