It’s such an important insecurity to talk about.
“You wanted him/her to be The One…” past tense is important. Why wouldn’t you have wanted that? And if the ex is a recent one from a significant relationship, still kinda sorta wanting it is pretty natural. As long as you’re honest to yourself and your partner about it and will realize and say it aloud if you’re not making progress with getting over that feeling.
“…and I’m not The One…” you might not be. But so what? People change over time. If you spend a precious year, or two, or thirty, with someone you really care about and experience wonderful things, but eventually your values shift into incompatible directions enough or whatever sudden crisis happens and you can’t stay together anymore, the end doesn’t devalue the journey you were on together.
“I’ll never live up to everything you wanted with him/her” why not focus on being everything you want to be with each other? Just overall, stop comparing yourself to others. Stop comparing yourself to the view you have of others, or the view of others you think others have (had.) You are you, you are right now, and that person is precious. That person must be precious to at the very least yourself for success. If you keep devaluing yourself, nobody else can appreciate you to the fullest.
So celebrate you. Celebrate your partner. Celebrate the beautiful mess you can make together.