Poorly spindly Mimosa tree- help!

Hello,
This mimosa tree has long been a joy in our family garden, currently about 45 tall. In recent years it has been so robust and strong and green it has needed pruning and keeping under control, and until about 2 years ago it was thriving with bouncy yellow flowers in winter.

Now it looks terrible and rather desperate and spindly- please see photo. We removed a bay tree growing near it in case it was competing but no improvement yet. The tree surgeon says he has seen many poorly Mimosa over the last couple of years.
Can anyone recommend a course of action for our lovely tree to recover its former glory? thank you in advance for any wisdom!

 

They are not totally hardy in my experience depending on species.
I assume you are somewhere mild in winter.

I also tend to think they are not long lived here in the UK mostly and dont reach a great age.