You’re not alone! It is estimated that over 20% of dogs struggle to be left alone and, given that they are a social species, this is not surprising.
Again, getting support from an ABTC Registered Clinical Animal Behaviourist would be best, as it is helpful to determine exactly what is going on for your dog at an individual level, so that you can get the best advice for your case as there can be multiple factors at play.
If your dog has been unhappy each time that they are left alone then repeating this (even for short periods) is likely to make the situation worse, rather than better. Instead, you need to do your very best to not leave your dog in the situation that they feel unsafe in and tackle it at their pace.
This is a challenge! You may need to find some friends/ family or neighbours that can stay with your dog while you go out for short periods for now. It’s good that your dog is happy to go out to playschool – you may need to make more use of this while you work on the issue in the longer term. You will then need to gradually teach your dog that they’re safe when left alone and that you will be coming back – this is rarely a quick fix.
There are some good resources here, which you might find helpful: www.subthresholdtraining.com. In terms of your dog struggling even when someone else is with them, I would work on encouraging them to build positive relationships with other people who stay with them so that they become a more trusted figure who your dog can rely on to help them to feel safe.
Having them play with your dog, do scentwork together and engaging them in fun, positive, reward-based training are all really good ways to do this.