You will find, as you read others experiences here, that some questions about her behavior will never be able to be answered - even if she were alive and moved on elsewhere. It can be very frustrating.
The use of meth was most likely a way to self-medicate as a way to escape her reality, even though the 'reality' entered into when one is on meth is outside the norm. Many people with BPD do have their highs and lows. They are used to it, and so perhaps she was using the meth to bring her out of a low, or make her high even more extreme. Overall, it's not a healthy way to self-medicate.
Part of the grieving process is going through a few stages. You can read more about them in our Lessons:
https://bpdfamily.com/message_board/index.php?topic=136462.0 - denial, anger, bargaining, depression and finally acceptance. It's not a straight path to acceptance, so you may find yourself going in and out of these stages.
Keep posting here... . you are not alone. We will all help you get through this.