I Had Some Bad Experiences In Friendships And Dating Relations

What should I do? I’ve been with this guy for almost 3 years we broke up after I found him with another girl in our bed in our house I feel hurt and betrayed and I don’t know what to do? To some people 3 years get over yourself but I can’t I’ve known this guy for 8 years we are best friends for that long and got together he knew me inside and out and I knew him inside and out I feel very upset :(