Originally there was 1 group of Europeans speaking a Proto-Slavic language, before the Great Migration, but this was long after the established existence of the Romans and their dominions. The Romans were much older and the empire was so big that the language had many dialects spoken far and wide. Vulgar Latin, from which the modern Romance languages come, was spoken far and wide long before Slavic tribes split apart and spread all over Central, Southern, and Eastern Europe, and West Asia. Proto-Slavic broke into West, South, and East Slavic. Christianity was accepted only in the 9th Century, almost a millennium after the Romans encountered it. Christianity is what brought writing to the Slavic people. The Old Church Slavonic was used and understood by the Bulgarians, the Русь (Rus'), and Serbs. The West Slavs accepted the Roman Catholic religion, and the Latin alphabet of writing began to be used by them. West Slavic is what Czech, Slovak, and Polish, come from. East Slavic is what Ukrainian, Belarusian, and Russian come from. South Slavic is what Serbo-Croatian, Slovenian, Bulgarian, and Macedonian, come from. This all happened recently enough in history that the Slavic languages still share some mutual intelligibility. The same could be said about Romance languages, but they've had more time to diverge. I learned 3 years of Spanish in school, and I can still understand a lot of mostly written Italian and Portuguese and a little French
Meanwhile you can think about "se" as about English reflexive pronouns, e.g. "oneself", "-self", "-selves" (myself, yourself, himself/herself/itself, yourselves, ourselves, themselves).
And about "si" as about "for oneself / for myself" (zpívám si)
In most cases ;-)