There is only one thing that causes unhappiness. It's attachment. Attachments to a specific person, place, time, job or a lifestyle.
Attachments are what make it so hard to accept change. Once we have them, we don't want to let go. We become slaves to maintaining the status quo.
But everything is in a constant state of change. We have certain things for a while and then lose them. The only permanent thing is our reasoned choice. They are resilient and adaptable. The sooner we become aware of this the better. The easier it will be to accept and adapt to what does happen.