I’ve just seen someone I haven’t seen or spoken too in 13y and it’s given me a bit of a shock

[IHopeYouStepOnALegoPiece]

Something happened 13y ago and although they didn’t do it, how they reacted made everything even worse. It almost broke me and I didn’t ever talk about it again after her reaction.

I thought I saw them last week for the first time but wasn’t sure but it definitely was them today. I feel really strange, on the edge of a huge cry, white hot rage angry and very panicky

I don’t know how to pull myself out of it

It’s ok to feel like that. Crying releases stress hormones. Give yourself time to react, cry if you need to. Then in half an hour, an hour, whatever, pick yourself up, give yourself a treat (you deserve one after a day like today) and carry on. Repeat ad infinitum.
Disclaimer: it might work, it might not, but that’s what helps me. I’ve been in a similar situation after being raped: people’s reactions were in some ways harder to deal with than what happened. Every so often something or someone brings it all back. This is my strategy for those times.