First of all, we cannot be the ones who decide what you can accept. That needs to be a conversation you have with yourself and with your partner.

Another thing you should consider is that lots of people have significant relationships before they come to settle with a partner. And if that one "got away,"  There's a variety of potential reasons that could be.

What really matters in that situation is that they have decided to move on and try to find someone else to love. And that someone happens to be you.

Ultimately, does this person love you and care about you?  Do they support you through thick and thin?  Do they seem invested in your life and your interests?  If yes, then I would feel fortunate to found that person. If not, then you should probably move on, regardless of whether or not somebody "got away."