Welcome and hello
You're doing a really good job focusing on keeping your friendships alive, and that is great. And she is trying her best to support you.
It sounds like she does well when you give her advance warning and structure. This is probably true for many of us, and definitely true for many people with BPD. When you do tell her that you'll be spending time with other people, it may also help to refer to a time when the two of you will be together, like when you get back you'll do xyz together and you're looking forward to it. Tell her what you're going to do, remind her a few times, then remind her again the morning of, then again before you go. She needs more reassurance than someone without BPD.
She is likely to feel needy in ways that you can never satiate. It's part of radically accepting her for who she is. At the same time, you can never merge into her, the way her disorder makes her compelled to want. In fact, she needs you to have boundaries more than anything, even if asserting your boundaries is difficult for her.
There are times she may be in a regressed state and feel more child-like, and you leaving her will feel the same as if a parent is leaving her -- very scary. I have found it helps, in addition to the things above (advance warning, validating the desire to spend time together later), if I appeal to the adult-like side of her, that she handles things much better. For example, "That time you said it was important for me to see friends was so thoughtful. You know how to be supportive, even when it feels hard."
That kind of thing.
Then, when you are out, do your best to not respond to text messages if she tries to stay connected during your absence. When you set a boundary, it's important you are consistent and clear. You can discuss her text message when you get back, and if it's a boundary to not check texts while you're out with friends, then be clear about it and let her know what you ARE willing to do (like text her when you are in the car headed home, a nice courtesy).