Welcome @Matchalattexo .
Grief and loss is painful. I guess it is natural to question what could have been done differently, but in this I would also encourage you to also consider the persons autonomy and own responsibilities. If someone tries to hide something from you, is it your fault for not knowing it was there? I don't think it is.
I know I hid my own mental illness for years from people - very successfully. I don't hold any blame on them for not knowing. I hid it because I didn't want them to know. And by doing this, I took away their option to try to help me.
Not sure if this perspective helps at all -- if not, please ignore it.
Either way - the community here is here for you, to talk to, to share your thoughts and worries, and to listen to you. And if you want, to answer questions you may have, or explore ideas of what may help you process feelings/thoughts.
@tyme (above reply for you from MatchaLatte)