Oh dear, I am so sorry for what you are going through.
I know this pain too well as I have been in love with someone for over a year and I also am married. What helps me is:
Reading about other people who go through the same - there are lots of threads here about people being heartbroken. You will see that you are not alone in your suffering. It helps a lot to see other people's perspectives on the same problem.
Keep busy. Friends, new hobbies, running, entering competitions - anything to keep your mind occupied.
Learn to love yourself. This I found very helpful. Think what a great, cool and wonderful lady you are. There are so many small and big things about you that people like and admire. You are a catch and you are fabulous.
Be kind to yourself. You are allowed to feel what you feel. There is no shame and nothing wrong with loving someone. Don't feel bad and guilty. Accept your feelings and they will become a part of you rather than something you are struggling with.
Maybe rethink your marriage? I found that imagining that I really can leave my DH, if I so wish, has helped me to be more rational about the whole thing. Actually leaving him would be difficult as I have DC and only a part time job and no family nearby. And so I felt trapped. However, if you imagine that you have a real option of leaving, that you can work out all the practicalities - eventually - made me realise that, given a free choice - I would still stay with my DH. And then I took it from there..
Above all - lots of hugs to you. This is so hard and it's not something you can discuss with your friends and family. So we suffer alone whilst pretending we are fine. But you are not alone - You will find that there are plenty of us saddos :)