Some of us suffer from an excessively high sense of obligation to others - making it difficult to opt out or say no to all the requests made on our time.
Here are some simple tricks to help you stand your ground.
Be pleasant and polite - just say 'sorry, but I'm already booked'. It doesn't matter if that 'booking' is a quiet night in with the cat. It's your time and you are entitled to spend it how you wish. You don’t have to explain why you are not available; you just have to state that you are not available.
If they insist, thank them for the invitation and repeat your 'sorry but I'm already booked' statement (You might have to do it several times if they are persistent. All you have to do is outlast them!)
Don't let anyone make you feel guilty when you've chosen to do something else. You matter too.
You can offer a rain check for another time that suits better (but only if you want to).
Enjoy whatever you have chosen to do!