Sorry you are potentially being evicted from your home, OP. This must be very difficult for you.
Your landlord has the right to sell the property to whoever he wants for whatever reason. But he has to follow the correct legal processes if he wants to evict you.
He can not just “tell” you that you have to be out by the 20 May. He has to serve you written notice on the correct day and give you at least 2 months to leave the property. If he has not done this, do not say anything to him. It is in your interests if he fails to follow due process. It means the start of the process is delayed. Even when he has served you correct notice you do not actually have to leave on the leaving date. You can stay put and he will have to go to the courts to evict you. This will take time. When he gets the court order, which he will if he has followed the correct process, you can still refuse to leave. He then has to go back to the courts for a second order which will be enforced. At very best, the process will take six months. Possibly longer given Covid delays. Shelter can advise you on the details.
In the interim you do not have to allow potential buyers or surveyors in to the property unless this is specified in your letting agreement.
It might help to start looking for somewhere to go. Depending on your age, your Council may be able to help you but as a single person you are unlikely to be a high priority. Get yourself on the Council waiting list and bid for anything that comes up. Contact local housing associations. Ask family and friends if they could accommodate you. There are a lot of people facing higher bills at the moment and they might want/be forced to rent out a room or rooms to a lodger - and an older, single, female lodger can be a good bet.
Can you look around for extra work? Some families with older relatives look for sleep in night carers for example - one month on one month off or similar. Could you do paid babysitting?
I think being proactive and having a list of things to do may help with the stress you are facing. I am sorry you find yourself in this position.