I have full custody due to my ex being a drug addict, now my oldest is asking for specifics about why mommy is not able to see her, looking for advice on how to proceed.

I am a father of three girls, 8, 6 and 4. I took full custody of them back in April of 2017. In all that time they have not seen their mother once. She is a very bad heroin addict and nobody even really knows where she is, nor does she attempt to see the kids.In all that time, my six and four year old have completely adjusted with no questions or apparent curiosity about where their mother is or even why they can’t see her. I think they were young enough when everything happened that they just rolled with it. And they are very, very happy where we are now.My eight year old on the other hand has a deep and keen interest in her mother and her whereabouts. She doesn’t know exactly why mommy isn’t here but she has some idea. We have a family member with schizophrenia and when my daughter was very young she asked why Terry acted the way he did I explained that his brain was sick. About three weeks after I took custody of them (and 6 months of being kept from me while watching their mother’s behavior deteriorate into severe drug abuse) she asked me “Why can’t we see mommy? Is Mommy’s brain sick?” Even more than a year later I can’t think about that without almost bursting into tears.Mommy’s brain is sick and it is true after a sense so I told her yes, and we discussed that it wasn’t contagious, it wasn’t like cancer and it was different than schizophrenia. Until now that satisfied her. Mommy can’t be here because her brain is sick.Today she asked my mother (who watches them when I am at work; thank god for you, mom) how mommy’s brain was sick and why it was sick and some other pointed questions. She has finally become curious enough to need specifics, maybe to help fill with knowledge the emptiness from the lack of a mother she feels.This is where I feel almost totally out of my depth and so, so lost. I will never lie to them, and I know that it is important that I answer the question and not put her off. It will only make her seek the information elsewhere. I also want her to always feel like she can come to me for help and answers.Have any of you ever been through this? I am terrified that opening up and telling her the truth that mommy is a heroin addict will do some damage to her, cause some emotional damage. She is an old soul for her age but I don’t know if this is a piece of information that can benefit her. She is only eight fucking years old!Thanks for reading and as lost as I feel and as scared of the talk we will have tomorrow, I will appreciate any and all advice you all can give. via /r/Parenting https://ift.tt/2RtxBBU