I wouldn't say this although it is correctly formed... Could you explain the adjective a bit more? Secretive perhaps? But more natural in finnish would be to say something like I am not telling you everything or I keep things to myself...
Sorry I have been away and did not see all the messages to the english translation unfortunately, because this is a bit tricky again.
disguise - naamioidun? (=be behind a mask) too concrete I think....
pretend - olen olevinani? (=pretend to be better than I am, too proud)
pretend, fake - teeskentelen? (=pretend to be someone else or trying to be different from what I am, cheating in that way)
Have to think about this a little bit more, i'll be back tonight.
Sorry, I'll repeat: my finnish sucks. I think it's better reject when they are wrong.
I won't waste your time again, Maribel.
Thanks for your patience.