That's untrue. Sevastopol is the main port and naval base in the Black Sea, but Russia has plenty of others, and some are even undergoing expansion - Novorosiisk ( the backup one). Sevastopol is in Crimea, but was shared under an agreement between the Russian and Ukrainian navies. So in reality, Russia gained nothing there besides an extra front for their current invasion of Ukraine.
Romania, Bulgaria doesn't border Russia , Turkey is not on the land border and they have decent relations with them. But sure, they're likely not happy with that either - they just can't and will not do anything about it. With Crimea/Ukraine - they could.
I have never seen someone claim that Novorosiisk was a sufficient port. Maybe it's a capacity issue?
> Sevastopol is in Crimea, but was shared under an agreement between the Russian and Ukrainian navies.
It is in a foreign country, an unfriendly one. That greatly limits Russia's freedom of action.
Putin view Ukraine as a natural part of the Russian Empire - they are his vassals.
They can't have a free and prosperous Ukraine that has 'gone to the side of the west'.
The Crimean peninsula is just a bonus.
Hitler yes. Napoleon no. The French neither attacked nor retreated through what is now Ukraine.
That "Mystery of Russian Soul" lies among the lines of "Kiev - mother of Russian cities" (term of 12th century Primary Chronicle) and too many other historical facts. Like Wild Steppe (south of Ukraine) were fought for and liberated from Crimean Khanate by Russian Army. Modern Kherson, Nikolayev, Sevastopol, and Ekaterinoslav(Dnipro/Dnepropetrovsk), were established by Gregory Potemkin (Empress Catherine times) and populated by peasants from Russian heartlands - Tambov, Ryazan, Pskov, etc.
All that is well known and well remembered facts by Russian public. So we should not expect that Russian people will be so against all that. If in doubt then see the above.
Without Ukraine's Crimean peninsula, Russia loses sea access to the Black Sea and thus the Mediterranean for their navy (and also for trade), which historically has been a primary determinant of Russian military influence in Europe. Otherwise, they only have northern ports which are frozen and easily choked off (and ports way over on the Pacific - imagine having to sail your navy around Eurasia in order to attack or defend).
More importantly, Ukraine's geography is easy to move large forces across, in that respect a major battlefield (with due respect to Ukrainians). Napoleon and the Nazis both crossed through that region on their way to Russia, and when Russia attacks West, they go through Ukraine. Russian wants to control it and to prevent others from controlling it.
By controlling Ukraine, Russia can put more pressure on, have more influence over, Ukraine's neighbors, including Poland, and further cut off Latvia, Lithuania, and Estonia from their NATO allies.
Finally, Putin wants glory - don't underestimate the personal characteristics, such as ego, of unaccountable dictators - and to recreate the Soviet empire and the illusion of power. An independent, democratic Ukraine both undermines that appearance (and perception is everything for dictators) and embarasses Putin's claim to power as an authoritarian dictator.