You have to keep talking to people. Get a job(or hobby, etc if you are already in a set career) where you talk to a decent amount of people but the conversations are low pressure.
You most likely aren't up to par socially but people don't care much at all because they have their own shit going on. If you consistently talk through the day and do it 5 days a week you get a sort of social momentum and will be able to objectively notice the change in yourself after months of this. I have noticed myself being able to just say most of what I want to say with a clearer head. And most of all being able to fully listen to people and comprehend what they are saying so I can respond appropriately without being too short and can keep a conversation going without blanking out.
I took a job at a car dealership where you constantly are having to speak to people and I even was a shuttle driver for them where you are literally sitting next to a customer driving them and have to talk. Shuttling people I feel like was the best place to learn to keep a conversation going even if it could get quiet when I first started. You get to where it's so much less work to keep a conversation going.

It's work though. You literally have to practice being social. Outgoing people can talk a lot because they have been practicing talking their whole life because they haven't had anything to hinder their ability to practice being social.