1)
Invest in your story. Well, it's not a sure way to stop you from creating a new one, but investing time and effort in your work will prevent you from dropping it completely. (You may stop writing it for the meantime, but because of your investment, you'll eventually return to it.)
2)
Focus on your work. One of the reasons I self-isolate (avoiding playing games, reading mangas and other novels, watching anime, even temporarily leaving group chats) while writing is so that my ideas 'saved' during the times I'm taking a break won't be 'influenced' by other works. I mean, it's not uncommon for us to find 'better' ideas while doing those things I mentioned, hence I keep myself off from it until I finished my manuscript.
3)
Avoid belittling your work. Yeah, we know our faults and we are aware of what needs to improve. However, constantly saying that 'my work is shit' will eventually influence how you perceive your work. Then, once a 'better' concept/idea comes in, nothing will stop you from dropping it. Believe in your story; it's your brain-child in the first place. And the fact that you already wrote something makes you better than others who keep on thinking of ideas over and over again.
4)
Self-discipline. One of the best thread replies I read here personifies writing as a 'muse' that we need to work hard on in order to gain 'her' cooperation. I agree to this. Without self-discipline, you will be stagnant. Not writing anything will never develop your skills, as well as constantly 'trying to improve' before you actually wrote something down. For short, quit thinking you 'can't write', and start writing. Then, correct and develop your style as you go along.
5)
Create something that you love. It's hard to keep your readers' expectations, but it's even more difficult to give time and effort for something you are forced to do. That's why, there are a lot of authors here who give advice to 'love your work/create something you love'; after all, at the end of the hype, only your 'loyal readers' will remain...those who see potential in you and your story.