i) The Reformed emphasis on the assurance of salvation is in reaction to Rome, which denies the assurance of salvation. In traditional Catholic theology, you constantly walk a tightrope between dying in a state of grace and dying in a state of mortal sin. Salvation or damnation becomes a matter of lucky or unlucky timing.
So Protestant theologians emphasized the assurance of salvation to counter that error. That, however, can lead to an overreaction or overemphasis, as if we're supposed to indulge in morbid introspection. Spiritual hypochondria.
ii) There's a sense in which the assurance of salvation is overrated. It's important to reject a theological system that denies the possible assurance of salvation.
But having a sense of assurance doesn't mean you're heavenbound, and not having a sense of assurance doesn't mean you're hellbound.
A sense of assurance doesn't make you saved, lack of assurance doesn't make you unsaved. The presence or absence of assurance doesn't change the reality.
It's like the possibility that I was born with an undiagnosed genetic defect that will cause me to develop a degenerative illness in my 20s-40s. But I won't know if that's true unless and until it happens.
It would be unreasonable to let that hypothetical possibility haunt me. Rob me of happiness because I fear the dim possibility that I might develop a condition which ruins my life. That becomes a self-fulfilling prophecy, where it's my fear of something–which in all probability will never occur–that ruins my life, rather than the thing I fear.
Suppose I never marry or have kids for fear I might possibly have this ticking timebomb in my system. It's not the degenerative condition that makes me miserable, but the nagging fear. I may never develop that condition. Indeed, it is highly unlikely that I will develop that condition. Yet I deny myself the happy life I might have had–with the wife and kids and white picket fence–for fear I might not have a happy life! (in the unlikely event that I develop this imaginary condition).
If I develop that condition, I will be miserable, and so I avoid a normal social life just in case I develop that condition, but it's my reaction that makes me miserable, and not the specter of the would-be genetic defect.
It's a mistake for people to fret over the assurance of salvation. Just avoid doing things that are damnable!
ii) In addition, some people are prone to depression, which makes them more susceptible to spiritual self-doubt, because that's just a reflection of their general self-doubt. And that can be a vicious cycle. It's depressing to be depressed! And there's the fatalistic sense that even if you shake off depression, it's waiting for you just around the corner. You can't put it behind you, because it lies in wait to jump you when you round the corner.
Depression intensifies foreboding about the assurance of salvation, and vice versa.
But as I say, the assurance of salvation is often overblown. Like fearing the possibility that you're born with an undiagnosed genetic defect. You keep looking for symptoms. When you're not feeling well, you wonder if this is the onset of the dreaded degenerative condition–even though there's no evidence that you have a genetic defect. Even though that's statistically improbable.
iii) There's a certain paradox about spiritual self-examination. The people who need it don't do it and the people who do it don't need it.
By that I mean, there are spiritually self-confident people who are overly confident. And that's obvious to bystanders. There are spiritually self-confident people who are poised for a downfall. Ironically, their excessive self-assurance is the catalyst for their downfall. Others can see it coming, but they can't.
They are the high-risk group. And they are the very people who don't feel the need to examine themselves.
I think concern over assurance of salvation is mainly of value to people who are spiritually complacent. A check on one extreme.
But for normal Christians, I don't think it's necessary or beneficial to be too self-conscious.