Ex and best friend

My girlfriend and I had been friends since high school,we have all the same friends and partied together from 16-20. Got together at 21 and were together for 10 yrs.she broke up with me after I broke a promise regarding drugs.We both used just y methods. Ididn't take it well and felt abandoned and continued my use for few mths. We seen each other near daily so apart from obvious didn't seem real,I still felt like a couple. It took me a year to clean my act telling her numerous times I still wanted together thru year.I could never find the courage top ask after a year never the right time and was scared if she said no that would be us. It's been 2.5yrs and thought we both been with no one so maybe still chance. I found out 2 weeks ago that 7 mths after we broke up she was sleeping with my best friend and all our friends knew.no one told me only found out after his dad slipped up at pub and told my flatmate. I feel betrayed by everyone. She knew my feelings never changed for her,knew I wanted her back so did he because I talked to no one but him about my feelings. He used everything I told him to get into her then sat and listened and helped me when I was drunk enough to tell how I felt.

How the f*$k do I get over/thru this