The practice of letting go of negative thoughts is honed on the philosophy that only you can control your actions, no one is making you do anything. It is about choosing to respond to the actions of others, not reacting. So if you feel immense anger at your partner for their actions, then feel that anger. But do not use that anger as a stepping stone to reacting in the moment by lashing out. Rather, let your thoughts run wild in an area/room where it would not hurt anyone or you. Once you’ve cooled down as your emotions rose and fell like the tidal waves at sea, it is then that you can calmly assess the situation to decide what your next actions might be. If you were hurt, then communicate that hurt. The key is in your communications, not your reactions.
‘Letting go’ means to feel then move on, it is not letting others off the hook for their actions.