When it comes to having unpredictable energy and not being able to commit to activities outside the house, it is really helpful to remember to be gentle with yourself.
Remember that you aren't trying on purpose to ruin a trip by backing out at the last second. Know that you aren't trying on purpose to make your partner miss out on family events. When you don't have the energy and health to do something, step outside of yourself and see yourself as a concerned friend would see you. You would have compassion on yourself, and you'd say, "Hey, you! You are such a fighter, such a brave and strong person. You do everything you can (and then some) with the energy you have, but you don't need to overpush yourself. Be gentle to yourself."