Bonjour and welcome to the Showcase, Opur.

This is a beautiful first song here.
I like the dark mood and the instruments/arrangement you chose to emphasize this mood.
You have a beautiful voice that is perfect for this kind of song.

The mix and production are pretty good overall.
The only thing I would change is the reverb on your voice, which is a little too much (but still suits the song well).
However, I would work on the sibilants in the reverb tail a little more.
The easiest way is not to use the reverb as an insert on your vocal track (where you wouldn't be able to EQ the sibilants without also changing your vocals), but to send the vocal track to an FX channel (or whatever it's called in REASON), insert the reverb there, and reduce the sibilants with an EQ.

Anyway, it's a good song [Linked Image - Only viewable when logged in]
Enjoyed my listen!