cheated on my long term, long distance boyfriend

I've been with my boyfriend for around 4 years. Around 9 months ago I had to move to Asia for work, and his work kept him in the UK. The last time I saw him was around 6 months ago and due to the time difference and our work schedule we rarely get chance to talk. This is not to say my feelings toward him have changed, I still love him and he is my best friend, but as difficult as it is to admit, I have cheated on him.
I've felt attractions to people before (its natural) but I've never acted on it whilst in a relationship, however I get on really well with someone I work with whom I happen to find attractive, and to cut a long story short, we were drinking and before I knew it we were kissing. I knew I shouldve stopped it at the time but it felt so good and honestly I didn't want to. One thing led to another and we ended up sleeping together. This happened on his last night of working at the company as he has moved, so I know it won't happen again.
However, I don't know if its worse that I have more than just an attraction to him or not and every time I think of my boyfriend I feel ridiculously guilty and like I should tell him. I don't want to lose him and I know I've done wrong and no one should be cheated on, but some selfish part of me wants to keep it a secret to keep us together. But then I think surely our long distance can't be working if I've done this? I'm really confused and if anyone could offer any advice it would be so well appreciated.