aio for blocking my best friend on everything

overall, this is childish for our respective ages and i know that.
i, f20 met my ex best friend f22 at uni at the start of last year. we got really close quickly because we had a lot in common, such as style music and our experiences with EDs.
a mutual friend of ours had recently been horrible to me, manipulating me and lying to me about my friends, and then blocking me on everything out of nowhere and accusing me of things i didn't do.
i made it clear to my best friend that kind of behaviour was disrespectful to me, i was hurt, and that i didn’t want to interact with that friend anymore. point blank period.
i found out a few weeks ago that she decided to reconcile with that friend. i was hurt, but i figured i can’t tell her who she can and cannot be friends with so i said nothing but decided i was going to step back from the friendship a little bit as i was bothered by it and didn’t want to engage in ED discussion with her anymore anyways.
another friend of ours was having a party and had invited the ex friend, so i told my best friend that i wouldn’t be coming as i would be drinking and knew that there was a chance if something happening. she proceeded to ask me 3 times to go anyways after i said no 3 times. i was later told the friend who was nasty to me wouldn’t be there, so i went but she was and targeted me and started unnecessary drama.
some things happened and i posted a story saying i was relieved that i hadn’t gone to a event because that ex friend was there and would start drama with me, and i was upset that people made me feel difficult for trying to avoid her, and my best friend replied, ignoring my emotions and stating she’d never noticed anything like that.
after which i explained to her that i didn’t want to talk about it with her since i knew her stance on that friend but i didn’t appreciate that she didn’t respect mine. she then started to blame me for not telling her, even though when i did she kept asking me to hang out with that friend so i didn’t bring up the topic again. she accused me of being hostile and bad vibes in my messages even though she also said i was talking to her the same way i always do. when i told her i found that offensive she said it was “whatever” and that since she didn’t mean it as an insult i shouldn’t be hurt.
we have a lot of mutual friends so even though i was upset about this incident, i didn’t say anything to anyone bc i didn’t want to make them feel uncomfortable. i later found out through some friends that she was messaging them unprovoked to tell them that we had fallen out and that i had made her upset, and she posted a story calling me sick for removing her from my close friends story without realising i could see it.
this irritated me because i hadn’t spoken to anyone about our issues and yet she was bashing me on social media and to my friends. i blocked her on everything because i simply had just had enough of the going behind my back, but my mum is saying i was too hasty.

You were reacting to repeated boundary violations and escalating behaviour. You were clear early on about what you needed to feel safe, you didn’t try to control who she was friends with, and you even chose distance quietly rather than confrontation. She repeatedly pushed you into situations you said you weren’t comfortable with, dismissed your feelings when you expressed them, reframed herself as the victim, and then involved other people and social media instead of speaking to you directly. That pattern matters more than any single incident. Blocking her wasn’t impulsive so much as a final boundary after private communication failed and things started happening behind your back. Your mum may be thinking in terms of reconciliation or optics, but you’re allowed to protect your peace, especially when someone is misrepresenting you and stirring things up. You can always unblock later if you genuinely want to talk, but taking space when trust is gone is reasonable, not dramatic. The healthiest thing now is to stay neutral with mutual friends, don’t defend yourself excessively, and let your consistency speak for itself.

It appears that at no point in your extremely long post do you seem to indicate that at any time before the final argument you ever actually talked about this with the other friend (simply stating that you are effectively "embargoing" the other person I don't think really counts as talking about the situation of how you intend to navigate the fact they are still friends with them) or, more importantly, considered how they felt being caught between two people who were both their friend, who didn't like each other.
The lack of empathy for your (now former) friend's position, and expecting them to simply know how you feel and adjust their behaviour to do what makes you feel best (without changing any of your behaviour) is unreasonable and rather self-centred. The entire situation may well have been avoided had you simply had a frank and adult conversation with them about your feelings and their feelings and come to a mutual agreement about how to proceed.
What you did do instead is the kind of behaviour I'd expect from a 14 year old. Honestly you have a lot of maturing to do in my opinion. You also should recognise that in a friendship you do need to accept that your friends aren't always going to everything how you like to be or centred around your needs, and that your friends also have needs (and yes, sometimes you can't deliver all of those yourself too).
Also the final response was a significant overreaction relative to the "inciting incident" which seems to be basically, your friend continuing to invite you to events and be friends with you while being friends with someone you don't like.

I did talk to her about this previous to the final argument actually, she didn’t like this friend either until she suddenly turned around and reconciled. She told me that what the friend had done to me was wrong at the time, so when she said the reconciled I maintained the same thing I said during the initial fall out—I didn’t want to or be around that person because I was hurt. This ‘embargo’ didn’t come out of nowhere.

I totally understand that we have different needs. I told her those needs and it’s not like I was telling her not to breathe, I was just saying I wouldn’t go to something if that friend was there. And she repeatedly asked me to go to the same event over and over after I said no. I’m not sure that me going to this event is one of her ‘needs’

I’m genuinely curious about what a mutual agreement would have been in this case. I didn’t say anything abt her being friend with that girl-I have an issue with it but that’s her piece and not my place to speak on.

The inciting incident is my friend ignoring my boundaries, calling me names, and then spreading that information to our mutual friends unprovoked. Not the argument itself.

In hindsight I actually don’t think you read my ‘extremely long post’.