DD was like this a LOT when she was younger. She has improved, still has her moments but its not so constant as it was. But even now, it is still "right back in the moment" when she thinks about incidents, and it is hard for her (age 11) to focus on the whys and what happened rather than simply being angry about it.
One thing that really helped her was to reduce her overall stress levels. Sounds odd. But it's a bit like, she had a petrol tank of ability to cope - when she was stressed, that took a lot of the petrol so she only had a small bit left over to deal with any new thing happening so meltdowns came very quickly. But when her overall life was calmer and more relaxed, she used less petrol dealing with the everyday stuff so had more petrol when she needed the boost from someone else winning or hearing an extra loud noise that hurt her ears or having to do something difficult in class....
We did things like tried to make sure that when she was having fussy times with food, we didn't focus on that but made sure there was some food she would eat. She was allowed to go into a quiet space away from the bustle of home life, cool and dark and calm, either to chill normally or especially when meltdowns were happening. We worked on relaxation techniques with her, and did things like rubbing her back (soothing) or letting her have long baths and playing with her rubber ducks (water soothes a lot - but sometimes showers can feel like needles on her skin so baths tended to be better). We tried to have clothes that were comfortable to her - soft tracksuits, cut out labels, seamfree underwear etc. Fidget toys to get anxious energy out. And reassuring her that we loved her lots - spending time with her, getting her to help with chores working together, reading to her in bed even long after others her age were not getting stories, even now she listens to audio books or talk radio in bed, that sort of thing.
Parties were problems for a long while. We found she wanted to go but they were hard for her. So we went to some but not others, choosing smaller group ones (not generally whole class ones), activity ones which weren't competitive (but fairy princess ones would be hated so more sports type activities), and those of her particular friends. And we'd let the parents know to call us early if need be (we'd often hang around locally if we thought there may be issues). Sometimes, and having forewarned the party parents, we'd arrive late or plan to leave early.
And for her own parties, we learned to outsource. So holding the party somewhere away from home, someone else "ran" the party, and both DH and I would be there - 1 would be minding everyone else, and 1 would be minding DD. So if any other child needed a drink, the loo, whatever, that parent did all those jobs, and organizing food or party bags or whatever else was needed. The other parent was able to so some stuff, but if DD needed a few minutes she came to that parent, and would leave the room for a walk, maybe get a drink from a dispenser machine rather than her bottle inside the room, or just sit on that parent's lap for a few minutes - just able to offload from the noise and the bustle of everyone else. "Allowed" to duck out for a few minutes, being told in advance she could, and then being able to go back in when she felt ready - so the petrol tank (of energy to cope with everything) was refilled before it got empty. And that made a huge difference as she KNEW herself she could leave, and that there was someone there just for her (ok I had more to do as DH wasn't there to put out plates and be ready when they came up from the sports hall - but DD enjoyed her day and so did her friends as there were no meltdowns!).