2003 Chevy Monte Carlo Won't change gears

My Monte Carlo takes a long time for me to shift gears with. I can rev up to 6000rpm and it goes 30mph. I have been driving it in this condition for about a month. It is getting progressively worse. Is this a trani issue or something else?

By the way I changed my spark plugs but I only changed three(gotta get a puller for the back 3). Does this have anything to do with it?

Thanks