The decision to move to mastering is a personal one, IMHO. If you are happy with it, then yes, it is time to push it through the mastering process.
There are some nitpicky things, like the sound of the pick hitting the strings on the acoustic is a bit loud to me in certain spots - may be a personal taste thing. But overall, I think the mix sounds fine.
Do you have something against reverb on vocals? If not, I would use some. Especially the backing vocs. They kind of POP! in and are very dry. Slow songs like this just cry for verb. I think it would help the voc sit a little better.
Could use a bit more compression on Voc too. Just a wee outta control in a few spots. And maybe a de-esser. There are some sibilance issues. Pretty bad at 3:56
I would suggest trying a multi delay rather than reverb for the vocal. Nothing heavy, just enough to smear the sound out a bit. I don't dig the sound of the lead guitar at all, too much of the surrounding walls and not enough direct sound. It's really funny sounding. I hear you going for an eric clapton sound and that aint it.
Some of the keyboards seem to want to be heard a bit more in my opinion....but I can't tell because I can't really hear em. The acoustic sounds too picky and not thick enough to hold the quieter passages up.