Wife falling for another man how to let her go?

Ok I know this sad I have to use internet but tell you truth I have no one else turn to. Me and my wife Samantha have been together for 7 half years. When I met her she had nothing in terms of belongings and a place to call home. For the last year half we been having problems. She says she tried for 6 months to be happy. Problems was our relationship went stale. Reason for that was we both have worked whole time been together for last 2 years we have worked opposite shifts. Me 8 to 4 her 4 pm to 1 am at pizza place. She started to get depressed cause we used to go out all the time. then it started to taper of. Reason one is we have three kids. The opposite shifts posed a problem cause we could not spend much time together. We only got one day off a week together. She is iron anemic so she tired all the time. Also with work we both all ways tired. When she started to get depressed I asked her what problem was. She told me but I failed even after her telling me several times. My reason for is several things. One was I was so upset that she was depressed because of me I kept loosing focus on her problem. Also around time we started having problems was when we both started smoking meth. We have now been clean for a month but relationship still not going good. Meth caused me to not pay attention pretty much. She said she tried so hard so long while being unhappy she resorted to speaking with a guy at work. She says that he has in synk perception better then anyone she has ever met. Yes i know i failed at time to listed I realize now but it too late. She told me while we were together that she was falling for him. Reason was he listen. She says he makes her feel good about her self. I fail to do that. she my first love. She then tell me she kissed him while we where together. More then Twice and passionately. When she told me she wanted to be with him. I was gonna move out let her keep everything even tho its all ours. She then pretty much said she wanted me out so he could live there. At first I said ok then I changed my mind. That's my house my family. When she thought he was getting moved in she was thrilled then I changed my mind. She then realized what she was doing wasn't right. During all this he kept saying stuff like when he leaving so I can come be with you. He was plotting with her where to send me. He a 27 year old living with grandma runs across my wife who needs a sh older and comforts her. For one he disrespected me by kissing her knowing she was married. For two he didn't even have nerve to stop say hay you married. If he is doing this now whats to say he wont do it when he finds something better after being with her. Instead of doing it respect full way he was being anal about it. Frankly I think he Turing to get out of his grandma house by seeing she needed sh older he was there and actually having feelings for her. You think he just looking for way out of life living with grandma by using here. Did the disrespect fullness show he not genuine. I m afraid he just going to mooch off her. I warned her and she sorta took his side. Saying to me you just dint understand. Im the type Id never cheat on my wife not even a kiss. I want raised that way. My wife says she wasn't either but claimed that I was good thing that happened to her but then she goes and kisses him behind my back. Will it happen again. I know I did wrong ignored here feeling and problems for a while. I realize now but to late. She says she tried for long time but gave up. You think he just out for change life or real life. My main ? is should I let her go forever. Ill all ways be there for here now. She did say her feeling for me are stronger because of kids and what we have been Thur. You think she just thrilled someone else was interested. MY ? is how to i let her got either temp or for good.