I agree with Rose. If you really think it's over with your wife then end the marriage if that's what you feel is right.

Then you can look at another relationship.

If you initiate with this other woman now she may well react negatively due to the fact you're married. Even if she reacted positively this would leave you all in a very messy situation. In any divorce proceding your wife could use this behaviour against you.

I personally would be wary of thinking a new relationship will make you happy. I suspect much of your discontent is with yourself, it often is, are you midlife by any chance?