English Translation
I’m decisive.
Apparently “decisivo” can also be used for being “decisive”, though oddly enough for the “io” form, no sentences exist using this adjective on Tatoeba.
For “you”, and other people both versions do appear:
E.g. here and here
or for formal “you” here and here.
Also note the subtle difference between the current sentence, where “deciso” is an adjective, and “Mi sono deciso” (which comes from “decidersi” - “to make up one’s mind”).
Furthermore, this sentence is also of course different from “ho deciso”.