You need to get her to see you going out as a good thing (or at least not a bad thing).
You need to build her being left alone gradually. Don't just go out one day for five hours.
Get her a Kong and hide her favourite food in it.
As you are getting ready to go out tell her what you're doing and be consistent with this. Say something like "Mummy's going to work" and say this every time she will be left alone. She'll know what it means and what's expected from her.
Then give her the Kong and leave. Don't fuss her, don't coo, just leave.
The first time, go out for about 10 minutes then come back.
When you come back, fuss her but don't go over the top. Give her a biscuit and a stroke and tell her she's a good girl.
Next time, do the same but leave her alone for longer. Keep doing this, gradually increasing the length of time you're out.
She will learn that (a) you always come back (b) she always get s treat when you come back and (c) she always gets something nice to eat/play with when you're out.
I'd recommend changing up the treat/toy as well so it's not always the same Kong with the same food in it.
When I go out, I leave lots of little toys dotted all around the house for my dog to find and then figure out how to get the food out. As soon as she sees me putting on my shoes now, she gets really excited because she knows it means I'm going out!