Hi there,
Anxiety can be tricky to treat and may require some medication so the dog can respond to behavioural therapy so you may want to talk to someone who can come out and work one on one with your dog (eg. veterinary behaviourist)
Google behavioural enrichment or environmental enrichment to see some good ideas for keeping your dog busy during the day. It is really commonplace for zoo animals but seems to get overlooked a bit for domestic pets.
Also if you feel she is starting to ramp up her anxiety as you start to leave the house you can change your leaving routine. For example dogs can learn whether you are going for a short time or a long time (eg. if you put on your joggers or take the washing basket out the back as compared to picking up your handbag and keys) and a change in their expectations of what is going to happen can help. You can also do some interval training where you basically give treats for being quiet (this works best with a clicker but you may need some help to get started with clicker training). Once the dog expects treats for being quiet you can build up to longer times, walking out the gate, walking around the block etc. So the dog learns the you being away is followed by you coming back and a reward. But it is also good like Christy said to give something like a bone when you leave to distract them as well.
Lastly if you think another dog would help but don't want to commit to getting a new pet you can consider fostering which provides a temporary home for a dog without the long-term responsibility. Your local rescue service like the RSPCA usually need people to foster animals.
I hope this helps.