I am not proud of what I'm going to admit, but I was having an affair with a married man for the last 7 months... and he was just tragically killed when a car ran into him on his motorcycle. He was a truly wonderful man who lightened up any room when he walked in. There was about 1000 people at his funeral to celebrate his life - he seemed to know everyone! He left behind a wife of almost 30 years, a daughter and two grandkids whom he loved so much. I too am in a long-term relationship, so it's been hard to hide the grief that I am suffering. They were friends as well.
I am the secret that no one really knows about, so it's extremely difficult to talk about what I'm going through and just how much I cared for this man. He was 48 years old - far to young to die. We had an intimate relationship and shared so many laughs and... just the little stuff... we just made each other happy for the moments we were together. It pains me that he is no longer here and we will never share a moment together again. I feel deep sadness for his wife and daughter and can't even imagine what they are going through.
I don't even know why I'm writing this on here, but I just don't know what to do with myself. I feel so completely lost. How do I cope in this situation?

In his memory, why not you do something good for someone or take up a cause that both of you liked...

I think this grief you are feeling could be used to your advantage,
I am not trying to be callous,but I do think you should start valueing your own life and your (long term )relationships more.

Death being an inevitability, it was his time to leave this world.
You still have your own(long term) relationship, why not try to give that relationship a chance and work on that.

He is gone and his family has their friends and family to help them cope.
And as you know you cannot help them in their grief.

Feeling sad and grieving is natural,but as you said it is hard to grieve when no one knows why you are grieving and you cannot share with anyone.
Cry,grieve and then Let Go!
Restart evaluating your own priorities and what you wish to be and where you wish to go from here.
The sad truth is that your lost love will not return,but you are still living and life is so valuable.

You sound like you are doing quite well, it could be worse remember. Make sure you keep the secret, secret. Do you have photo's of him? Make yourself a picture plaque on the wall dedicated to him.
Also, try not to go for married men. I do with you the best.