To suffer is to be reminded over and over again of something negative that either has happened, is happening, or we believe will happen. Something is perceived as “negative” if we think it will make life more difficult than necessary.
Physical suffering and mental suffering aren’t so different. Physical suffering is experienced when a pain signal is sent from a damaged or irritated part of the body to the brain. The signal is sent over and over again, reminding us that there’s a problem. That’s what we do to ourselves when we experience mental suffering — we remind ourselves over and over again of a perceived problem, whether real or not.
In Buddhism they say suffering is the result of desire. There’s a lot of truth to that, and it includes the undesirable — the things we don’t want to happen and the things we don’t want to be true.