GF of 7 years cheated

Hello,

So as the title says, my gf of 7 years cheated on me. I M25 and her F24 have been together since senior year of HS which would be 2017 for us. I fell in love with her the day i met her and my love for her has grown exponentially since then. We have 2 beautiful daughters a 5 year old and a 1 year old. I think life has been good but for the past 8 or so months our relationship has gotten extremely rocky. I do 1 thing she doesn’t like and she threatens to leave me this happens often and i am in no means perfect at all. She expects certain things from me like helping around the house and more with the kids and i have gotten very lazy this past year so i understood where she was coming from. Fast forward to November of this year she starts going out with her cousins alot more and for context her cousins have never liked me and they always tell her how she can do better. So the date is Dec 10th we wake up in the morning to get our kids ready for daycare and she tells me to change my 1 year olds diaper and i make a face which wasnt right i was just super tired and didnt feel like doing it. She gets extremely angry and tells me that we are done and sends me a long text in the middle of the day reiterating that we are done. We dont talk much the rest of that week until 2 days ago when i went through her phone and found a group chat with her and her cousins. She told them that we were done and how she was happy to leave me. That shocked me bc i didnt think she was serious so i beg her to give me another chance and after a long talk she says she will think ab it as long as i change. Later that night she’s downstairs so i go through her apple watch to read more of the group chat and i find that her cousins have set her up with a friend of thiers and shes been talking to him a bit. I confront her and she tells me its nothing and that she wouldnt talk to him anymore and blocks him. So yesterday we actually had a great day we were both happy but i was still worried about this guy but she kept reassuring me that it was nothing and that she loved me and all she wanted to do was be with me and make our relationship work and how shed never hurt me. I was about to go to sleep when i saw her phone next to her while she was sleeping so not beinh able to help myself i look at that group chat once again and the farther i scroll up i see screenshots of her and the guy talking and apparently when she goes to her cousins house he is there and in 1 message her cousin tells her to have sex with him and that they should be together. Naturally im hysterical i wake her up and she denies it and refuses to tell me anything she kept telling me it was nothing until I left the house to go for a drive to gather my thoughts. She calls me and begs me to come back and when i finally do she tells me she will answer any questions i have and the first thing i ask her is if she had sex with him and she said she did. She told me they had sex in his car this past Saturday which was a week ago. Apparently he ghosted her and she knows she freaked up. I am naturally heart broken and dont know what to do i love her so much and i wish this could go away. I want to give her another chance but im afraid it will happen again since her cousins are always hyping her up to do bad stuff. I need advice

UPDATE

She has to work this morning so im alone with our kids and this is the worst feeling ever. I have never felt more broken and i know everyone is saying it will get better but it doesn’t feel like that. I feel like im going to explode so im going to call her mom and let her know what happened and if she has any advice but as of right now i am done. I know its fresh but it has consumed my mind and i wish i had relatives to leave my kids with for a couple of hours so i can really gather my thoughts. Thank you all for the replys i will update along my new Journey.