So sorry to hear you are struggling, but completely understandable. I always think that no matter how knowledgeable and well read you are, nothing really prepares you for having to talk about such very sad and difficult things with your own children. But to reassure you, children in general and very little ones like your DD in particular are so resilient and almost blase about topics we find really awful as adults, even things like death, loss and illness, just because they don't really understand what they are, so they take their cues from the adults around them and if things are not presented as bad news they are unlikely to be actively upset - but they do take in what they are told and are processing it all the time - making it so much easier later than a big 'announcement' when they are older. The most important thing for her to know is that she is safe and you love her, which I am sure she 100% does 
For what it's worth (and I am no expert so please feel free to ignore), I would start talking to her about it now in a 'drip feed' sort of way, don't feel obliged to tell the whole 'story' at once or to use the word 'adoption' if that is hard for you (after all she doesn't know what it means so no need to use it until you are more comfortable). You could break it down into lots of little chunks, so if she starts to be curious about where babies come from (as they often do at that age especially if you know anyone who is having a baby) you can talk to her about how some babies grow in their mummies tummies, and some grow in other ladies tummies (some people like to talk about 'tummy mummies', I don't personally as I think DC only have one mummy but as you prefer) and then go and live with their mummy (and daddy if applicable) forever.
You can then move on to talk about how her brothers and sisters grew in your tummy, but she grew in X (whatever you want to call her BM)'s tummy, but she then came to live with you forever, and you were all sooo happy because you love her so much etc etc. Make it a more positive thing about how lovely it is that she lives with you rather than sad she doesn't live with BM. I think it's good to emphasise also the 'forever family' part in case she gets confused and thinks she could move again and have another mummy/family.
Then perhaps a bit later when this message has sunk in and as/when she starts to question 'why', you can start to talk about how X (BM) loved her very much but wasn't able to keep her safe/wasn't able to look after her, and so that is why she came to live with you forever (and again how happy you are about it).
It can be much much later that you drip in some of the worse details about 'why' BM couldn't look after her, and again you can make it child appropriate - talking about how BM's own mummy and daddy didn't look after her very well and so she didn't know how to look after a baby well, didn't know what food a baby needs to eat, for instance - simple things she can understand. You could take the opportunity to do a bit of 'babying' with her yourself or playing with dolls and toys to model how you 'should' look after a baby which some DC really like, wrapping her up and 'feeding' her etc.
In the meantime while you are doing your 'dripping' perhaps you can make it a project to make a new life story book, perhaps using one of the less bad pics of BM or none at all, just words/drawings or pictures of your DD when younger if you have them, and then lots of nice pics of her, you and your family at the end. A lot of DC go through phases of not being interested at all in their life story book or not wanting to look at it at all so don't be surprised if she doesn't seem that bothered, but it is nice to have something there for them if they do want it later down the line...