Wife has found her soul mate

We have been married for 21 years and had 4 kids. Our common friends set us up by telling us one had a crush on each other. We dated and we somehow clicked and soon got married after graduation. We were happy and had a child a year after. We soon got busy with family and work and neglected our marriage. After 4 yrs, she had an affairs with a co worker who impregnated her. It was a very painful experience for me but we made up and she said sorry. I accepted her back and tried to forget about it. It helped that we moved away in a far place. We had two more children and lived as a happy family. Our children except one are doing poorly at school and was sometimes a cause of tension. She is a strict disciplinarian with a viscious tongue while I'm a liberal and happy go lucky type of guy. We often clash about values and how to raise our kids. A year ago, we bought a new house and it put a lot of stress in our relationship as we have different ideas and opinions about it. Then it happened, I noticed she was different. She became secretive, she was always up late in evening texting someone and seem very upset all the time. I found out that she is chatting with another guy, our engineer and it was very personal. Confronting them both they told me it was just friendship. I did not believe because the texts didn't seem like pure friendship. We quarreled about this many times and she said she is unhappy with me. It's been over a year now although I think the worse is over.. I still can't get over the thought that she still sees this person and even goes to him every other day to ride a bike, just the two of them while I join my male friend or my children and we ride our bikes too but in a different place. Lately I saw his guy friend wearing same bike apparel as my wife color and design that she bought for him. When asked why she said he asked her to buy one for him and he apparently paid for it she claimed. This naturally incensed me but for the sake of peace and harmony at home I let it pass. Our children knows that their parents are having a rocky marriage as they are sometimes witness to these bitter fights. They are all in their teens now. I have always feelings of insecurity, doubts and I can't restore the trust I have for her before. Contemplating of ending it but I'm sad for our children and I don't want them to have a broken family but deep inside I'm dying and always feel irrelevant and useless to my wife who has no remorse and even blames me why she strayed but wouldn't admit to any infidelity. I wish I know what to do and hoping I can find answers, insights from others' perspective. Thanks