I really reasonate with that. It's a really shitty feeling like you're sinking behind others and yes, having no concrete explanation makes it even worse.
I hate hearing about other people succeeding in life because every time I do, I think about how I'm not enjoying their success and it's a sharp jab to my self-worth. It shatters my mood for the rest of the day, maybe even week.
People think it's horrible to react like that and I wish I could be happier for people but I literally can't help it, the feelings just hit me. It's such an unimaginably soul-wrenching feeling that you're missing out on the joys of life that others get to enjoy.
I hate giving 'advice', but I have to say that I hope you do not accept any of the BS explanations some people will inevitably attempt to give you if you bring this up to them, unless they can REALLY back it up (they won't be able to). The real explanation is likely beyond all of us, extremely complex, and most importantly, bereft of any 'wrongdoing' on your part, because I don't think it can reasonably be denied that we exist in a universe where every effect follows unavoidably from a cause, and this chain of causes stretches back to a point far away from your knowledge or access.
Maybe the most important thing to recognise is that the future is open and none of us know what will happen going from here. But I don't know, because that uncertainty can also be distressing.