Can't get over a break-up

My girlfriend of 8 years (we started dating at 12) broke up with me around 3 months ago. She was my absolute everything. My best friend, someone I trust completely, I respected and admired her and I loved her. She said she just didn't feel the same any more. I have taken this really hard, 3 months down the line and my feelings for her haven't changed. I wake up and go to sleep thinking about her and this is really hurting me mentally. I have started a new job at one of the big four accounting firms and have lots of exams to do but thinking about her is stopping me working, I cannot think about anything else. I am also having issues getting myself motivated to work. I have never done things for myself, I worked hard (I have a degree in Mathematics) and got this job because I wanted my girlfriend (who I was going to propose to next year) to have/do anything she wanted. My family always worried about money and I never wanted her to have to worry about it and now she's gone I have no motivation to work hard at all. My family and friends all say I'm better off without her, they say someone else will be waiting around the corner but I cannot see it. All I see is her. To make things worse, during our time together we tried so many times to have sex. Every time it really hurt her and we always stopped (never did I push her into it or get angry at her, I was always supportive) so we went to the doctors together who said that it was just something natural that would go away over time so we had to be patient. I know this may seem stupid but I feel one way to express your love to someone is by having sex and we never got to do that. Now though, I know (3months later) she is in a relationship with someone (her brothers best friend) and is keeping it very secretive. She is also planning on going away with this guy on holiday. Just knowing she is close to someone is killing me inside and knowing she could be having sex with this guy hurts me so much. I just don't know what to do. I feel so helpless and lost without her.

I also want to add that before our break up I went through several months of exams where (I know this is wrong) I didn't give my girlfriend enough of my time. One thing that's going through my head is that maybe she sought out comfort in this guy. I say this because jumping from such a long relationship straight into another seems odd to me. Just my opinion though so I don't know.