Eyeamme summed it up--this is not for sissies! It is overwhelming, exhausting, and so discouraging dealing with a child with BPD. Are you seeing a therapist? The grief and despair you feel can and will get better over time and with help. My company has an EAP benefit that I used to get some free sessions with a therapist when things were really bad with my BPDd24.
Most psychotherapists have turned her away as a client, claiming they only deal with "functional" people who are not coping with something, but not with these sort of issues. Most professionals try to wipe their hands clean by saying I should just put her in a hospital.
Do you live near a major metropolitan area? We found better psychiatric help in DC which is about an hour from our home. There are likely to be more and better choices in a bigger city. There are also some long-term facilities that I wish we had known about when our d was underage and we could have made her go (and had our insurance cover some of it)
In my case, she is physically a big girl, mentally 18 with her arguments and putting me down. And yet as clingy as a 3 year old. She has shocking separation anxiety from me. She either follows me around literally all day, not giving me any space at all to even think! Other times she will sit in her room in the dark for 3 days claiming she is of no use. She avoids trying to do any basic life skill needs, like cooking or baking or reading or learning.
It isn't unusual for BPDs to be bright and talented, yet immature in many areas, and also to lack confidence to try and learn new things. My d actually once told my sister that she didn't know how to make a salad! She was 21 at the time!
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Most of all she doesn't want me to tell ANYONE about her situation, and even becomes angry if I should try to approach the subject of her diagnoses.
It seems reasonable for her to want to keep her problems private, but that doesn't mean you can ignore them. Perhaps instead of discussing her "diagnosis" with her you can approach certain behaviors, and set boundaries regarding them. The tools on the right can really help with communication and setting boundaries you can live with. In our case, our d chose to no longer live within the framework we laid out for her and is not at home any more.
Keep reading--look for other topics that speak to you in this section. The more I read other people's posts and the responses they get, they less alone I feel. Sending you hugs and prayers