Why am I so bad?

I do fine against computers when I practice my RTS, but I swear there are so many people that play that are in an entirely different league. When I get ranked and do Versus, does it at least pair us with people of similar MMRs? Also, any tips would be helpful. I swear I was good at this game when I was 14 and now that I’m 32 my brain won’t do the things.