Oh no! I'm so sorry his bird passed away. It happens. Both my beloved cat and dog were in my mother's care when I left for school. They both died that next year. It broke my heart but it wasn't her fault, not yours. This kind of thing happens all the time. It's hard, but it's not your fault.
You are not the worst parent ever. You are a survivor who has supported your children through awful times.
The best thing you can do now is to try to stop internalizing your abusers message that the abuse was your fault and that you are horrible. Your children need you to stop agreeing with him and start agreeing with the truth: you are a good mom who does her best to heal. You make mistakes, but you are a brave and kind mother who loves her kids deeply. You have had the courage that many don't - to leave your abuser and work your butt off in therapy.
You don't have to be mean to yourself too because the bird died or because of ANYTHING else that has happened.
:hug: