Things that let me know it's real this time:
Years ago, an interest in a person would have made me re-consider the future, give it a chance; see what could come out of it. But now, interest in a person doesn't change a thing. I'm not going to change my plans just because I think someone is "attractive" or "interested".

Physically, I'm affected by human contact, arms brushing up against each others' at dinner is not wasted on me. It's just different now. The bar for the test is much higher. Is being with this person worth wanting to live?

It used to be a matter of latching onto something, grabbing for a vine as I'm sliding off a cliff. But there's been a fundamental shift and I'm not grabbing for vines anymore. It's a matter of whether the cliff is there or not. Whether the appearance of a person, or this or that development in life, makes the cliff disappear in my mind. Before, I've always grabbed for vines that appeared, but nothing ever made the cliff go away.