The "love of your life" is a myth. Like earth being the centre of the universe, or us being the apple of god's eyes, or the existence of life after death. All myths we are so prone to believe because they shield us from the (otherwise apparent) horror of reality: the fact that as living organisms, we are essentially alone among other mostly self-serving organisms, insignificant in the grand scheme of things, and finite in time.

The reality in the case of "love" and "relationships" is that you current "significant other" is out there for herself first, her kids second, and some source of security third. She doesn't love you; she loves the fact that being with you make her feel good/loved/secured/protected/high-status, etc. And the day you don't make her feel that way anymore (which comes quickly), and that in the context of the list of priorities written above, the benefits of leaving you outweigh the benefits of staying, then she will have no reason to stay and rationalise that she doesn't love you, or ever did..

This does not mean that there is no chance of you ever growing old and dying together, it just means that you have no way to ensure that it will happen. And that if it does happen, then you've been bloody lucky.

Just like with the other myths, the only way to solve the dissonance between reality and your wants is to drop the delusion and face reality dead on. These myths are just myths; Reality is harsh; it's ok, we'll be fine anyway. Buddhists have been fighting the delusion of permanence for millenia, you can just as well fight the delusion of lifelong love. It just requires to repeat the mantra: "She's not yours; it's just your turn; enjoy it while it's your turn".