I think that when you're composing a song, arranging it, producing it ... what lacks in the end, when you're at the mixing stage is: perspective ...
It's not that you're able or not to mix it, only someone with fresh ears catches what the mix lacks instantly.
and you probably are over-focusing on things that are not so important, and don't see what's obvious. (since you know those tracks so well, and become less sensitive about them).
I think that finding a good mixing partner really is the best thing. you keep involved in the mix, but with a distance.
so in the end, it sounds like you, only better. but hiring a "mixer" ... I would not for my usual gigs, but if I get enough budget, I'd consider someone who can take my music to the point I can't.
Cheers !
V