Try one of those kongs that you fill with food and they have to chew it out. Freezing them makes them last longer. Give the Kong to her filled with her favourite stuff only when you’re leaving so it’s a special thing she only gets then. Then try leaving her with the Kong for only long enough that she doesn’t get upset (2 -5 mins given what you describe), then come back in and take the Kong back. Then slowly increase the time over the course of a few days. The important thing is to come back soon enough that she doesn’t get too stressed, yiu want to start associating you being gone with happy Kong chew time.
Great suggestions already. A couple additional ideas:
Get a crate liner (like a flattish cushion, usually with fake shearling.) and sleep on top of it for a couple of nights. That way the crate will smell like both you and Preston.
Kong is a great idea. I’d only let her have it in the crate.
Also, for Preston, keep the weasle on the ground from now on. You can still slither it around and exercise them, but his back is always going to be fragile. I know exactly how he feels! 
This is probably a first - but I’m going to defend Stoid here by saying it is quite likely these were rotten dogs because they were cocker spaniels, not because Stoid ruined them.
Our first dog was a cocker, and she was a devious, garbage-rustling, sneak-peeing-and-pooping mess.* All of our other dogs have been much better-behaved with far better personalities. Cockers were ruined by indiscriminate breeding decades ago, both in terms of conformation and behavior.
*In defense of Sally, I should mention that she was a decent retriever, a reasonably good watchdog and an excellent swimmer, whom we loved despite her many and glaring faults.
An investment in a good obedience program can work wonders and make life with a dog far more livable and enjoyable.
You care and you are trying. That makes you an awesome dog owner in my book. I have been involved with pit bull rescue and my local shelter for 6 years. I know what bad owners are like. You are not one.
You’ve already been given great advice. I don’t really have anything more to add.
Do you know the how many _____(dog breed)____does it take to change a lightbulb? Jokes? I only know three, one of which is for cockers.
So how many cockers does it take to change a lightbulb?
Who cares? I can pee in the dark.
And as long as I’m at it:
Golden retrievers:
The day is young, the sun is shining, we have our whole lives ahead of us and you’re worried about light bulbs!
And the best is Border Collies:
just one, and he’ll bring your wiring up to code while he’s at it.
Completely agree, Lanzy. I’m not sure I understand the point of it.
I don’t know what progress you might have made in the last two months Stoid, I’d just like to add that consistency and routine are imperative for anxious dogs.
They need to know the difference between you being gone for short, long, or indeterminate periods of time. You need to decide on key phrases to differentiate chunks of time for her, and use them unfailingly. Absolutely adhere to the time frame you’ve given Zusje - without exception. Do not make excuses for being away longer than you told her you would.
I have no personal experience with medication as have also been suggested, but even having had a Dalmatian with separation anxiety it had never crossed my mind to use them. I think it’s about trust more than anything. I think it’s about that beautifully evolving communication between you and your dog and I think prescription sedatives greatly interfere with that.
You know the foods that have a calmative effect - use them.
Zusje needs excercise, she needs to be finding things and knowing things and anticipating things outside your home. Playing ‘weasel’ or whatever might be good for her body, but it does nothing for her doggy soul when she’s still within the confines of the house. If you can only walk two blocks, then take her one block, multiple times a day. At around the same time.
You need to get her out of your bed.
You might be her ‘mum’ - but she’s a dog. She needs to know her greater environment. She needs to read her wee-mail. She doesn’t know enough about what goes on outside that door when you leave, and she’s not there to protect you. Also, in the heirarchy, she’s above you. So, since she’s the boss and she’s telling you by her behaviour that you’re not going anywhere, then what the fuck do you think you’re doing leaving?
No wonder she eats holes in the walls.
To me, inventing games for her to play at home is all well and good, but it’s much like putting a video on for a kid - you do it because it’s easy for you, not because it’s useful for the kid.
It’s great that you focus on the food. That must be all pretty rote by now, so turn all that focus onto her behaviour. The behaviour will always tell you the motivation or the trigger. Its up to you to get the result you want from that natural behaviour. Steer the behaviour, modify it, centre it and THEN you can start to change it. Dogs are complex animals but their needs are really very basic, so look at it all through Zusje’s eyes and quit the excuses.