Whilst plug-ins etc may help some dogs a tiny bit - the reality is that you will not get anywhere significant with them.
Every single time your dog is left and she gets upset, it is reconfirmed to her that being left is a terrible thing.
So, as pp have said, you have to start with the amount of being left that she can cope with and never increase the level until she is ready.
The reality is that probably means 10, 20, 30 seconds right now. For dogs that have severe worry at being left, 30 seocnds is too long. It might even be you don't leave the room at all, just open the door as if to leave and then step back in. The starting point is dictated by the dog. When she can cope with, without being worried.
Pop her in a room, leave the room, wait 10 seconds, go straight back in the room. Repeat, repeat, repeat over different days, different times, different situations. It'll send you crazy with boredom and the ideal is that she is also bored. So bored of watching you coming and going that she barely even looks up.
Then in increase the time you are out of the room. 10 seconds becomes 20 seconds, becomes 30, 45, 60, 2 mins.
If she ever looks nervous or worried then drop back down a level and repeat some more, before slowly building up again.
When you have slowly built up to leaving her for a few minites in a room then start to put a coat on but not leave the house.
Coat and shoes.
Coat and shoes and approaching the front door.
Coat and shoes and stepping outside the front door.
Coat and shoes and staying outisde the front door for a few seconds.
You get the idea 
It's not an easy task and I totally understand why most people, when faced with this, instead jut arrange their lives so that the dog is never left. It takes multiple repetitions (sometimes hundreds) and patience and time.
Other things that help support this process:
- looking at more general ways to destress the dog, e.g. sniffing games can have a calming affect on dogs so introduce those
- training, using the brain but also learning important skills such as to lie on a mat for a period of time before being released; over time this can be worked up to lying there even when you briefly leave a room
- erradicating other stressors in her life, such as making sure she only gets fussed if and when she wants or doesn't go on the types of walks she might not like etc