People leave, even when they assure you multiple times that they won't.
You're not saying they're at fault for leaving; circumstances change, people outgrow each other, there's distance or simply life gets in the way. They're not in the wrong, but you start thinking maybe you're at fault for believing in them, believing that someone would be willing to be by your side in the tough moments, that someone would understand that physical distance never means emotional distance or that just because you can't do something for them at that time, you wont do anything for them ever.
People leave because they forget that a spark can initiate a bond, but staying is a conscious decision. You decide to be in someone's life and that's something you choose every day.
It's very easy to give up and easier to find someone better, because in a world with 7 billion people - there is always going to be someone better. It takes courage and strength to decide to be there for another human being and share a lifetime with them. It's not just romantic relationships that need such a commitment, it's every kind of relationship there is.
The idea of forever is flawed, because you dont get a forever. But you do have a now, and eventually you would reach a kind of forever if you keep each other in your nows.
So if you find yourself doubting the people you're around, don't be the one to give up on them, the feelings that made you want to be a part of their life remain but get buried in the monotony of living. Just remember that each day you're choosing them, they're choosing you too and now is the only forever you have.