Seneca's observation is not a joke. It is a precise clinical finding that predates cognitive behavioural therapy by two thousand years.
The Yoga Sutras identify five modifications of the mind-stuff, of which one — Vikalpa, imagination — is the specific source of suffering Seneca is pointing at. Vikalpa is the mind's capacity to create scenarios that have no corresponding reality — possible futures, imagined responses, constructed narratives about what someone said or meant or will do.
Most of what the anxious mind is processing is Vikalpa. Not actual current conditions but projected future conditions that may never arrive — and even if they arrive, will never arrive in precisely the form the imagination has constructed.
The Mandukya Upanishad's analysis of the dream state is relevant here. In the dream, the suffering is completely real — the fear, the loss, the threat all registered as genuinely as in waking experience. The Mandukya's point: much of waking-state suffering has the same quality as dream-suffering — constructed by the mind from memory and imagination rather than from actual present-moment conditions. This is not to minimise genuine difficulty. It is to make a precise diagnostic distinction that changes what intervention is appropriate.
The Stoic exercise: for any current anxiety, ask three questions in sequence. Is this happening now? Is it certain to happen? If it happens, will it be as bad as the imagination is currently constructing it?
In most cases: no, not certain, and no. The present moment, examined without the overlay of imagined futures, is almost always more workable than the anxiety about it suggests.