You would like to preserve the relationship, but you cannot or do not want to attend. A useful decline needs two things: a clear answer and a tone that fits the invitation.
‘Thank you for inviting me. I won’t be able to join this time, but I hope you have a lovely evening’ can be enough. It acknowledges the gesture without leaving the host to interpret a vague maybe.
You do not owe a detailed account of every private reason. If you offer an explanation, keep it truthful and proportionate. An invented excuse can create complications later and make a simple boundary harder to maintain.
Offer another time only if you intend to follow through. ‘Let’s meet next week’ is a new commitment, not a required ingredient of politeness. If you would like to reconnect but cannot plan yet, say that accurately.
For an invitation involving bookings, reply early enough for the host to adjust where possible. Practical consideration can communicate warmth as clearly as the wording itself.
Before sending, check whether your answer is actually a no. If the recipient could reasonably think you might still attend, make the decision clearer. Kindness includes giving other people information they can use to make their own plans.