I have been in a similar situation, I had been friends with a man 10 years older than myself, he had no kids but is engaged to another girl.
Our friendship became more of a relationship, even though I was always fully aware that he had another girlfriend.
I loved, well, still love him. He said he loved me to. I believe that he did love me. He was the first man I had ever been with sexually, and I will never ever regret that. I will never regret our 'relationship' because he made me feel very happy, and feel good about myself.
It is hard to move on, but all I really want is for him to be happy, I know I would not have made him happy really I cause too much stress. I have a bf now, not someone I have to share with anybody else.
I am not sure you ever really stop loving people you had once loved, but time makes things easier, so does not seeing them, or their partner!
Nobody deserves to be second best, you will find someone who will love you and care for you above all others.
Send me a pm if you ever want to chat. =) Take Care