You may want to read some of the posts from folks who have been cheated on or left for another person in the Coping with Infidelity portion of the site.
You're having an emotional affair with this woman and sound like you're in a VERY thick affair fog.
Of course you feel the new woman is so strong - it's called "lust"!
The new girl is 'shiny, new & exciting'.
She comes without baggage, without the ups downs, and boring domesticity that a marriage, or long-term relationship has.
You say that your marriage has been struggling for the last year? That'd tie in with the 7 year itch. Your marriage is bogged down with homelife & predictability... any new 'connection' will seem more potent.
I understand that your wife has some problems, but please re-read what you wrote. You are effectively blaming your wife for your difficulties... this is typical of someone looking for an excuse to cheat. You are giving yourself permission to emotionally check out and pursue new skirt, because you want to.
Lots of people cheat & leave spouses for other people. But, if you were like that, then why post here? Why not just bail out, destroy your family & set up home with this bright new toy? I guess by posting on here, you must at some level, know it isn't the right thing to do.
If you genuinely don't love your wife anymore, then the *right* thing to do would be, would be to end your marriage & *stop* all contact with this other girl too. You should leave your marriage and be alone.
This girl represents one of two things; either she is an exit affair, where you want to leave your marriage, but don't want to be alone. Or, you are in such deep lust, that you are not thinking clearly. Both of these can only be disastrous.
I really don't think you love this girl. You're infatuated with her - yes, but you don't love her. You want to be with her, so you're demonising your wife, and emotionally distancing yourself from her.
My honest opinion would be to stop all contact with this other girl. Then try to rekindle the 'sparks' with your wife. Try to focus all this romantic feelings on her.
If you honestly cannot love your wife anymore, then leave her, be alone & act with integrity.
Your post didn't sound that way though. It didn't start with how unhappy your marriage was. It sounds that there are issues, rather than do the "sickness & health" bit, you meet another girl, see "sparks" and feel she is your soulmate. Please think about what love really means. It means sharing history, children, bad & good times... it does not mean "sparks".