Warning: Long-winded, almost stream of consciousness, first-draft analysis follows.
I see the Romulans as a resembling the Roman Empire during ascent (before the peak in 212); and also a bit like North Korea or China today.
There are numerous feuding aristocratic leaders. Each is juggling for position and power (this is how they are most similar to the late Roman Republic and early Roman Empire). Honors and position can be earned by military victories, etc.
Romulans can be brutal to those subject worlds who resist rule; but provide quite a bit of "civilizing influence" to those subject worlds who submit (again, compare to Rome's treatment of extra-italic peoples).
The Romulans see a "manifest destiny" to one day rule the galaxy. They see it as both inevitable, and necessary to protect their home, and Romulus. They are ruled by the Romulan Senate, consisting of all the aristocrats, and there is a Council of Romulans or some such consisting of all Romulan Citizens. Only ethnic Romulans have suffrage, and then only if they have served in some capacity to advance the cause of the Romulan Way (expanding to rule the galaxy).
The Romulans are paranoid and nationalistic (similar to North Korea today). In my thinking, they fought an early war with the Klingons (prior to first contact with the UFP by either), and have fought some pitched wars with several races close to Romulus (FASA has some interesting backstory on a civilization a little more advanced than the Romulans between Romulus and what is now the RNZ - the Romulans conquered and eradicated them in their first major interstellar war), and also on the far side of the Empire from the UFP. Their first-contact with Earth/UFP, in line with their history of having fought bloody wars with every significant space faring race they had encountered led them to strike first and assume a fight to the death with Earth/UFP (The Romulan War).
With the establishment of the Romulan Neutral Zone, the Romulans faced a semi-hostile Klingon Empire, a (to them) very hostile UFP, and a hostile race opposite the UFP. Surrounded on three sides, they naturally felt squeezed. They view the Klingons as a serious threat, but also as a useful tool to tie down the Federation. They view the Federation as a mortal
threat (much as how the US and USSR saw each other). While the UFP cannot be attacked outright, it can be infiltrated and damaged from within. Their policy was to play the UFP and the Klingons off each other in order to instigate war between them - allowing the Romulans to sweep both aside and move on towards their inevitable Galactic Rule. They did not understand the nature of the Organian Treaty between the UFP and the Klingon Empire (even if they knew it in detail, they did not entirely believe
it). The Khitomer Accords (ST:TUC) lead to a thawing of UFP-Klingon Empire relations, causing panic in the Romulan halls of power. This was their worst fear - the Klingons and the UFP joining forces against them. The attempt to derail the accords detailed in TUC was in part supported by the Romulans. The Tomed Incident and the attack upon Khitomer (leading to the death/capture of Worf's father) were both "last ditch" efforts to instigate war between the Klingons and the UFP, or at least derail the alliance.
Their return to visibility in the 2360s was the result of sufficient redirect of efforts and confidence in their increased ability to stand on equal footing with the UFP and Klingons. The Dominion War drew all three together staring a push towards greater cooperation among the three "Great Powers", possibly eventually leading towards a new expanded alliance that might one day defeat the Borg and cooperate to push collective frontiers out and establish a Galactic Republic.
So, if you are looking at Pre-TOS and early TOS Romulans, they are frightened and cornered by hostiles on all sides. If you are looking at mid TOS Romulans through the TOS Movies, they are playing the Federation and Klingons off each other to weaken both. If you are looking at the Post-Khitomer/Pre-TNG Romulans, they are again frightened and cornered by hostiles on all sides. TNG Romulans are not paralyzed by fear, but are afraid of the Federation and Klingons. DS9 Romulans (Dominion War) are more afraid of the instability brought by the Dominion into the power balance than they are of the Federation and the Klingons. They still do not TRUST the Federation or the Klingons, but their fear is much more tempered. With time, and lack of betrayal by the Federation and Klingons, the Romulans may come to trust them both, and all three powers come to not only a peaceful co-existence, but even cooperation in unifying their part of the Galaxy, if not the whole in a very long time.