psycho girl help

Right so this may be very confusing and a long story so i'm sorry in advance.
(last year)
Basically, in sixth form I had a girlfriend and we broke up just before exams because we were getting too stressed and we were distancing ourselves because of it. We didn't put much effort into the relationship at that point in time and i think we both were losing interest in each other by certain events. We pretty much had a mutual breakup and said that after exams, we'll see where we stand and continue it.
Flash forward to September and we both decided to do a gap year, we still aren't together. We don't talk much. Getting back together is pointless, I have no feelings for her and i'm with someone else. Baring in mind that we were gonna talk abt our relationship at the start of summer but we haven't had a single meaningful conversation since. I thought it was safe to say that we ended it for good and i sent her messages expressing how i felt and she agreed.
Flashforward again and now its Easter, my friends are hosting a get together of our friendship group and you can bring a plus one - it's by the beach. I bought by gf and everyone introduced themselves blah blah blah. I completely forgot abt the fact that my ex would be there and i only realized until i was faced to face with her. She looked completely different in a good way but i still felt nothing towards her.
We're walking around town and seeing as my GF has never been here, I thought i would just show her everything. My ex keeps bringing up stuff like "oh yeah do you remember when we did....together" or "this place was our favourite wasn't it". I tried to brush off these comments and just ignore them but then she just kept walking by me as if we're still together. After when we went home, me and my gf just talked abt what happened and both agrreed it was weird.
what do i do because the group is gonna meet up this weeknd too

I forgot to mention the fact that the reason i called her psycho is that shes sent my gf a couple of texts saying how we didn't actually breakup and how my gf is just a side piece stuff like that. She's also told my best mate that the reason that i'm with my gf is bc she reminds me of her - though i don't see the resemblance at all. On top of this, she's put up our pics together on her insta so it looks like we're still together. I've blocked her and my gf has too but it just makes me angry knowing that other people might get the idea that we are tovgether but we are not

ah right that's slightly different then... very strange