How do I build a sense of identity?

Hello everyone, I hope today is treating you all well. A little while back I posted here searching for advice on bringing up the possibility of BPD with my new psychiatrist, and I had my appointment a little while back. She ended up bringing up that possibility before me, and said she wants to work through a diagnosis of BPD.

Although it's not confirmed yet, she seemed very confident in her speculations, and deep down I have a feeling that as my appointments continue with her, I'll end up being diagnosed eventually. While I'm crushed about it, it feels good knowing that I'm finally being taken seriously, and am on the track of finding answers as to what's going on.

Sorry, I just wanted to give a little bit of background information here before I get into the main reason I'm posting here, which is seeking advice on building a sense of self. For as long as I can remember my sense of self has been unstable. To be blunt, I feel like I have no purpose in life, and struggle to feel like a person. I don't know my goals, what I like, what I dislike, who I am, what I truly want, etc. I know a few things, which is a start, but not much. Even then there's a good chance that it can all be changed if I bond with a person because I tend to "mirror" them. I think I'm using that term correctly, I apologize if in not.

I've tried to pick up new hobbies, explore new thing's, try new styles, etc, but nothing ever sticks. I get bored, lose motivation, and struggle to feel connected. I feel like I have a void inside me that I can't fill. The only time I've ever been able to sort of build a sense of self, is if I have someone to go off of, but of course that was never truly me, so when that connection ends, I crumble, and am back at square one.

For the past couple of months I've been obsessed with someone who admittedly, as embarrassed and ashamed I feel to admit this, I barely know, and have lost myself completely. It stared to fade for a moment, but now it's back. I've tried so hard to get over my obsession, but it just won't stop. I had a little bit of sense of who I was before this, but since I've started obsessing I've completely lost it. It's exhausting, I'm terrified, and I feel like I'm losing my mind.

I'm not sure why I started obsessing over them again, especially since usually when an obsession fades, I tend to stop caring about the person completely. I think it might be because I started to lose my sense of purpose as this obsession faded, and my mind panicked because I had nothing else to find it again if that makes sense? I felt that void coming back, and needed something to fill it, but didn't have anything else I guess? I'm not sure if I'm explaining this well, but to sum it up, I don't feel like a person, and don't feel like I have a purpose unless I have someone I can build myself off of. I just can't seem to build an identity by myself.

I'm sorry, I don't mean to talk like people as if they're objects, because I know they're not, and I don't mean it that way. I'm just struggling with wording this because I'm still trying to figure everything out. I'm in a lot of pain right now. I feel lost, and don't know what to do to fix this. Does anyone have any advice?