Denise,
Was the end of the relationship mutual? If not, who ended the relationship? You or the other person? If it was you, did the other person whole heartedly agree or did they just acquiesce?
If it was mutual, and the other person has not tried to contact you or will not reply to your efforts to contact them, just know that, in time, the memories will fade. The feelings will subside. They may never go away entirely. Memories may still make you nostalgic, but you will not go on desiring this relationship forever. You will find someone who will take their place. Each relationship has it's highs and lows. It's positive and negative aspects.
If you have not tried contacting this person and it has been months since ending the relationship, you may want to take a break in the current relationship and find out if the person you ended the relationship with, is available. You may want to revisit the reasons you broke up in the first place. Maybe you've grown in some way, that will allow the relationship to be more satisfying now. At the least a break would allow you to clear your head, so that you can be honest.
It is not fair to the new person you are in a relationship with, to regret ending the last relationship and go on longing to be with that person. Your heart is not really free to be with this person if your heart longs for another. It is selfish to pretend just so that you can be with someone, and not be alone.
I wish you the best with this tough situation. Honesty is always the best policy. We need to be true to ourselves and be truthful with others.
Best Regards,
Cherie
