Belarusian Nationalism dates back to the 19th Century, and it was the result of the unique blend of influences between the Poles, Lithuanians, and Russians. Apart from the obvious linguistic differences, the Belarusians were also religiously distinct from the Orthodox Russians, largely being followers of the Uniate Church. As Russia came to control the area, they would define the region - White Russia - as one of the three "Russian" identities - the others being Great Russian and Little Russian (Ukrainian).

Ukrainian nationalism was a more coherent force for a number of reasons - expatriate populations in Galicia, a "better" historical myth in the form of the Kyivan Rus and later Cossack Hetmanates, and their overall larger population. Belarus, on the other hand, lacked these historical points to seize on. Worse, aspects of their culture were considered direct threats to the Tsar's power, so there was a great effort to assimilate them as Russians. Significant effort was put into converting the Uniate Church in Belarus to Orthodox, as the Uniate Church was not subservient to the state in the way the Orthodox was. Similarly, moves made to enforce Russian as the official language hit Belarus particularly hard - while a few Ukrainian publications were able weather these efforts in the 1860s and '70s, Belarusian publications were unable to do so.

Because the national identity that was less resilient to Russification efforts than Ukrainian, it tends to fall by the wayside in larger histories. One of the larger names in Belarusian Nationalism - Kastuś Kalinoŭski - published his works in both Polish and Belarusian. He would take part in the January Uprising of 1863 and be executed for it, but the uprising was not for Belarusian independence - rather, for the restoration of the Polish-Lithuanian Commonwealth. And that seems to be the trend with Belarusian identity. Though present, it's something that doesn't really come up and you only see if you're going out of your way to look for it in histories up until the 20th Century.

Things changed in the 20th Century with the creation of the Belarusian SSR. Initially created by the Germans to pull the region away from Russia, it would carry on in several forms until being subsumed as part of the USSR. In one form, it was to be combined with Lithuania (Litbel), but in the aftermath of the Polish-Soviet War, Belarus was left as a lone rump state within the USSR.

During the interwar period, the Belarusian identity was preserved as distinct from Russian. Though there's a facade of Soviet idealism on it, it appears to have been more of a practical way to foster irredentism against Eastern Poland. When the Soviets occupied Eastern Poland, they used the Belarusians as part of their justification, and they incorporated much of the land they captured into the Belarusian SSR.

The Second World War would devastate Belarus, perhaps more so than any other Soviet republic. The War as it took place in Belarus - both with its extreme devastation and the high amount of partisan activity - would come to be mythologized. Coming out of WW2, Belarusian nationalism seemed to lean heavily on this. However, Russification efforts would continue through the later Soviet era. Khrushchev at one point refused the Belarusian SSR's leader's request to make a speech in Belarusian rather than Russian, and in the 1970s, things had degraded to the point that Minsk was unable to find a professor who could teach Belarusian history in the Belarusian language.

So, coming out of the Soviet Union, the Belarusian identity is somewhat weaker than others. Lacking the Kyivan origins that both Ukraine and Russia claim, its identity stems from the more recent Polish-Lithuanian Commonwealth, and as a result, has been a constant target for assimilation by the Russians. It came to the forefront as Belarus was legitimized in the form of a tool for meddling powers, and its "weaker" national myth would see them drawing more heavily on their experience in WW2 than others.

Hopefully this makes sense. If you're looking for a more in-depth overview, I'd suggest checking out

Lost Kingdom: The Quest for Empire and the Making of the Russian Nation by Serhii Plokhy

It's a history of the development of the Russian, Ukrainian, and Belarusian identities, with a particular focus on the latter two, and it's where I'm getting the bulk of this information.