first off, i have AS not BPD, so i don't belong here. nevertheless, i'll offer you my thoughts.
learn to meditate, if you haven't done so already. meditation provides a way for the mind to reset. you can't stop thinking about things by thinking about them. you have to stop thinking about them. only by stopping thinking about them can you find the space for more positive thoughts. the trick with meditation is to focus the mind on something else, be it breathing, a mantra, whatever, to the exclusion of everything else. by shutting down all external stimuli (the senses) and internal stimuli (memory) the mind starts to effectively float in its own inner peace. the experience can be very expansive, in as much as there can be a feeling of connection with life and the universe, almost as if you are one with everything. this has to be experienced. and this allows you to feel more refreshed and able to engage more constructively with life.
one of the problems people often have is that they try to force their lives along a certain path. they get obsessed with the destination, rather than the journey. every day is an opportunity to experience life. we decide for ourselves whether or not to enjoy that experience. if we always look at what we're missing, rather than what we've got, we're always going to be miserable rather than happy.
and i think people have rather a false understanding of happiness. true happiness is a state of contentment. it's when we don't feel that anything is missing in our lives because we're sufficiently self-reliant to recognise that we can meet our own very modest needs ourselves, rather than looking to other people or things for it.
do this and you may find your life opening up in new and unexpected directions. at the very least, you'll find it a lot more tolerable.
i found this website recently, which seems to me quite a good resource ...
https://www.pursuit-of-happiness.org/