Letting something go, sometimes, doesn’t mean you’re not responsible yet for that thing. You might still be accountable, but without any attachment, without being too demanding or critical of yourself and others.
This is good, but it might have an aftermath, where after letting something go, you might be left with disappointment, but then you might come to the understanding that disappointment is not on you - but from you. It regards to someone else, usually based on a behaviour that you’d expect to be different - thus disappointed by someone else.
All you need to do is to accept that we are only humans... and we are imperfect. Luckily imperfect.