If you still love her, you will have to win her affections back. Court your wife like you did before you were married. Open the door for her (car door too), bring her flowers, compliment her, surprise her with little gifts like a hand-picked bunch of flowers, tell her how much you love her, cook dinner for her occasionally. I'm sure you get the idea.
This ex she has fallen for is complimenting her, telling her how much he loves her, etc. So far, he's winning.
Most importantly, sit down with her and express how sorry you are that you have ignored her and not given her the attention she deserves. Make a vow that all that will change starting right then. Do this on a night when the children are at the grandparents, then take her to a lovely romantic dinner, buy her flowers, then finish the evening by spending the time cuddling on the couch.