Yes, Roberta, I think you are correct. There is work and conscious choice-making involved with remaining in a monogamous relationship for an extended period of time. If I thought always, “this is not permanent so why work at it?” then there wouldn’t be many long term relationships for me. This other person in your life may not want to do that work or conscious choice-making because he or she may have to look at some uncomfortable aspects of ‘self’ in order to stay close in the relationship. That person’s path is unique to that person, and it is his or her way. You may ache for some time in this period of loss.

You may wish to try an exercise I learned from Sue Fredrick’s book Bridges to Heaven, which is about grief and loss and how to heal.
Basically, the short version is to get into a meditative state through repetition of your mantra. Once your mind is clear, allow images of you and your departing partner together in the best times. Keep allowing the images and feelings to come until you are bursting full. Then, take your hands and place them palms in toward your heart.
Speak to your high self, higher power or to God–however you believe–and ask to have this pain removed from you as it no longer serves a positive purpose in your life. Stretch your open palms out in front of you and wait to feel the weight on them lifted away. (For this to happen, you must actually visually ‘see’ your grief as a ‘thing’.) Once you feel lighter and healed, thank the higher power/God for this gift of release. You may need to do this daily for a while, but over time, the pain will lessen.

The good news is if you do this for a while, your energy will rise to a higher level of vibration and you will attract someone who is on a path that better fits with yours.
Nothing is permanent, but there are better choices and worse choices when it comes to pairings. May the next of yours be the best ever!

Namaste!