When i Upload my skin and click use then get into a world my skin is still alex

Hello. 

I just bought Minecraft: Java Edition for my Mac Yesterday. It was running fine, i uploaded my skin to the Launcher then clicked use, but when i joined a single player world, my skin only showed as Alex. Then i tried to join 2 Servers (Hypixel & Minepixel) and they both came up with Authentication Error. 

I searched up what to do and a couple of people told me to run a Malware scan which i did and I managed to clear out all the unneeded Malware.

Its the next Morning, i've restarted my internet and refreshed Minecraft and im still having the issue.
Its really bugging me and i need someone to help me.