Just repeat to yourself: "This is just a game, I should really just relax!"
While Minecraft worlds are practically infinite, there are technical limitations & hard limits placed to maintain critical stability. On the PC version, you could travel 32,000 KM (that's 32 Million blocks) in any of the 4 cardinal directions from the world's Origin (0,0) before hitting the "world's end". that would make a Minecraft's world area 4,096 Million KM^2 (= 64,000 ^ 2) or 4,096 TRILLION blocks
For reference, the Earth itself is only 510.072 Million KM^2... making a fully-rendered Minecraft world a bit over 8 times larger than the Earth. HOWEVER, I do have to mention that a full-rendered Minecraft world would take up a crapload of storage space & most players & servers usually don't explore that far... generally sticking to a 100 KM^2 world (100 Million blocks, or 10,000 x 10,000 area)... which is still relatively large, but fairly manageable for servers.
In a scientific / philosophical sense, the world is curved among itself to a respectable limit. HOWEVER, the curved nature of the world & it's limit are not immediately perceived by the world's inhabitants. Only when we observe things outside the world's bounds that we can see it's limits.
HOWEVER, since the game is future a world of fiction, it doesn't have to obey the rules of our reality & such impossibilities can easily exist. As mentioned above, it's just a game... you should really just relax regarding such stuff.