She needs to be taught the skills for spending time alone, no amount of barricading her in or blocking her access to you is going to work unless she learns the skills she needs to cope. Basically what should help is a programme of desensitisation, starting with giving her something yummy like a stuffed kong or long lasting chew/bone that she will concentrate on and stepping away while she's busy.
If she's as bad as you say, you may need to start off just moving a couple of feet away from her, then very gradually move to the other side of the room, then the doorway, then through the threshold, etc, building up to being able to shut the door with her on the other side.
When you return to her, make sure it's no big deal. Don't specifically ignore her, you can say hi or whatever, just don't make a fuss. Walk in, quietly remove the treat and go about your business - it's useful to have a bag to unpack or put the kettle on or something when you do this.
Then you need to get her used to being the other side of a door to you while you have a cuppa, read a book or watch tv.
It will be slow progress and there will be a bit of two steps forwards, three steps back, but this does work if you keep at it consistently.
I feel your pain. I am home most of the time and have found it very difficult to get my pup used to me not being there for that reason. Having worked up to it, I now make sure he has at least an hour a day in the kitchen with our other dog and me elsewhere in the house and he'll sleep quietly while I do that, but I had to go through all the desensitsation stuff with him to reach this point and I still can't go out for longer than an hour, even after 10 months.
It may help to try a couple of things that are available to reduce stress levels in dogs. There's DAP collars and/or diffusers which mimic the pheromone given off by bitches when they feed their pups and induce the release of stress relieving hormones in the pups. DAP didn't really help with my pup, but he was found abandoned without his mum at less than 24 hours old, so perhaps that's why. What has worked with him is Pet Remedy. We use the spray, rather than the diffuser and it definitely relaxes him. Other options are a Thunder Shirt. Pets at Home sell them and honour the money back guarantee so if it doesn't help it won't cost you anything. You could also look into supplements like Zylkene (a milk derivative that's available via vets or online) that help relax anxious dogs, but these can work out very expensive, so it's worth seeing if you can help her without in the first instance.
If you want to get your head around Separation Anxiety, it's causes and treatments I highly recommend Nicole Wilde's book Don't Leave Me. It really helped me to understand and treat the SA my older boy developed when our other dog died last year.