You need to separate your business from your personal expenses (mortgage)
In theoty VAT should not be a problem as you are effectively a collection agent for the revenue and are just paying over what you have charged your customers. However, if your customers are late in paying your invoices, then you need to address this as it is causing you cash flow problems.
If your income has dropped significantly, then your income tax bill should be reduced accordingly.
Regarding your mortgage- if you are struggling to pay it, then it appears you are not paying yourself a sufficient "salary" out of your business. You need to assess your business to see is it viable to continue.