What you are experiencing is very hard on anyone emotionally. The years you are describing as living as a couple are years a person does a lot of growing and maturing. When a person is in their thirties they are in a new place, a different milestone along with a different hormonal stage as well.

The fact that you have even managed to sit down and write out what you shared is a big deal. It’s good that you found a therapist to help you. No one “just” knows how to deal with a major change like this after 15 years.

Do you have children?