So I'm a pretty together person - I bought a house by myself, I have a good job, I exercise every day, I eat well.

To answer your question, its called mental illness. My anxiety and need for control means I HAVE to be in control of everything all the time. I exercise because I struggle with eating disorders and I will have a panic attack and feel awful if I don't. I "eat well" on the outside but in reality I have to work weekly with a nutritionist to re-learn how to eat in a mentally healthy way. Im super stable, but it's because I rarely challenge myself or do things I'm scared of- I'm lean towards stagnant.

I feel like people who feel badly about themselves really miss how often other people's lives look great when they aren't, or don't see how barely they are holding it together or how unhealthy their motivations are.

There is no such thing as being behind in life. There's just where you are. Do therapy.