I've never posted on here before, as I just found this site today, so I don't know what the rules are, or what I'm supposed to be saying here. I've read a few posts and they are mostly from people much older than me who have been through multiple marriages or years of commitment to the BPD partner. I feel small because I only dated my ex for around 10 months and I still feel very deeply wounded by all of it.
Welcome SailorOnLand
You're expressing how you're feeling, so so far so good

But SOL... .you don't know how old I am... ... .I'm not old... I'm just a bit vintage

Age doesn't matter, the number of PD relationships doesn't matter, the length of the relationship doesn't matter. We're all here because a relationship with a pwBPD rocks your world and so does the aftermath. It's not a competition where the one with the longest relationship gets the most compassion or is most entitled to feel shattered. None of us can judge how much these relationships have affected us or can compare the effect.
I feel like I don't know what happened. Like the whole ten months was a dream, or a practice-run, or something weird like that. I feel, after breaking up with her a few days ago, like I'm re-entering the painful void that I was just starting to confront and deal with before my ex girlfriend came along. I feel like certain areas of my development as a person got put on hold for those ten months.
Maybe you needed this relationship because confronting yourself felt too painful and this relationship felt as the easy solution for your issues (I think it did for me). Maybe you needed this relationship because it made your issues more clear. Maybe both. So IMO your personal development didn't get put on hold. You took a detour that showed you more clearly what to work on and why.
I feel like the pain and turmoil that this relationship and the recent breakup have caused is re-triggering my own emotional wounds with feelings of abandonment, isolation, and loneliness.
It feels like that because that is exactly what happened.
I immediately after the break up started looking for a new partner. I went on dating websites and started watching porn again (I had a nasty addiction to porn when I was younger). It all felt so fake. Like a bandaid meant to stop a gushing fountain of blood. It feels like I'm running away from my pain. I have deep abandonment issues and fears of being alone and the relationship with my ex-girlfriend helped cover up those fears for a long time.
IMO immediately looking for a new partner and watching porn is running away from your pain. It helps cover the fears of deep abandonment and being alone now the relationship is over.
But now I just feel confused and overwhelmed. I don't understand what happened. It literally feels like ten months just flashed before my eyes in a single day. I feel like I lost a part of myself, or I gained something that I don't want. I'm afraid of living my life. I miss her and I want to be with her again but I know I can never repair a relationship that was broken from the beginning.
I feel like I don't even know how to respond to all the things that have happened in our relationship. From her killer rage that just came out of nowhere, to her feeling dangerously suicidal, to her manipulating me into taking the blame for problems that I had no part of, to her taking care of me and not allowing me to function as an individual, to her being very wise about all my problems and telling me about them as if she was some sage oracle who had all the answers to my life (to which I responded with amazement, more love and adoration, etc), to her being amazingly satisfying in bed, to her validating all my feelings and making me feel so special and unique, to her comforting me and wiping away my tears when I had abandonment issues triggered, to her being one of the most thoughtful, creative, funny, unique individuals I have ever met.
You feel like we all do or did
Because a relationship with a pwBPD rocks your world like no other. And so does the aftermath.
But you're able to describe your emotions very well and see your own issues. That's a important to start healing. And healing is not a linear process; you will bounce back to all of these emotions every now and then. But less frequent the further you get.
I feel so hurt that she has probably already forgotten about me
BPD is not amnesia. She hasn't forgotten about you. She just found out you are not the saviour to end her core pain and moved on to someone she now thinks will do that (of course he won't either). Doesn't mean she has forgotten about you. Doesn't mean she is happy.
I could use some advice on where to go from here. I have a therapist and I plan to talk to him about all of this.
That sounds like a solid plan.
Plus you found this site and board. You can post here, read other posts, put in your two pennies worth (or 2 cents), and of course read the Lessons part (above the threads). You can vent here, ask opinions, get inspired to think, etc
So where you go from here?
Through the pain and the anger and the lessons and the sadness and... all of it.
Hang in there