This is what happens in longer term relationships. There's risk.
Risk of that person leaving you. Risk of them dying young. Risk of your love withering on the vine.
It's okay for that to scare you, and if it truly and deeply concerns you, then it may very well be your subconscious telling you that things aren't completely right.
But I would also caution you that things don't have to be 100% perfect for them to work out.
See, a lot of us know the risks. We see them, and they concern us. But we go ahead anyway.
And we do our best to make it all work out. Sometimes, that's enough. Other times, it isn't.
Not knowing how the movie turns out doesn't dull your enjoyment of it, now, does it? Same thing in life.
PS Some of this can be a maturity thing, too. When you've got more life behind you than ahead of you, a lot of these fears start to seem kinda silly.