Mum is meeting ex-girlfriend

Hi guys, i still miss my ex, i've moved on a decent amount but recently i feel like i'm just taking steps backwards and the past week has just been horrible. I'm just here to ask for advice as im feeling lonely af. I've been split up for 3 months, it was a messy breakup which i ended for own personal reasons but now regret. My mum met my ex in august which i was so upset about but at the same time i wanted them to meet because i wanted to see if there was any chance of getting back together.
I messed up with this new girl i was talking to, i had a most minimal symptoms of covid and im not allowed in school and they think i can just 'catch up' like just ugh. then my mum says my ex texted her out the blue and asked how she was doing and now there making plans. i always get so conflicted because i dont know if its controlling to say no to them meating but its driving me insane because i still miss my ex. I feel so sad at the moment and i dont know what to do.

Youve just posted a stream of consciousness without properly explaining the context.
In any event its fine that they get on and as you dumped her then I think its fine for your mum to meet up and be friends.
You broke for a reason, so its best if you focus on the future.
If on the chance you want to get back then you should understand why you made the decision and that the reasons for doing so no longer exist otherwise you are rertreading old ground.

you see i dumped her but then i tried to get back and it didnt end up working, thats why im upset. i think its important to know breakups are hard both ends

Yes but notwithstanding the details I can see she is the one to be upset. Your upset stems from something you initiated. In any event what are you going to do tell your mum she cant meet with your ex?
How old are you both and how long were you together?

we're both 17 and we dated for 16 months

You should just move on.
You are expected to go through several relationships and at the time there was something not quite right.
Maybe when your mum comes back she will have said something, maybe not.