Firstly, there is no centre to the universe and the universe seems to continue indefinitely in all directions. It is best to imagine the universe as infinite in size.
Now we need to explain why the CMB appears to be a sphere surrounding us.
Let's imagine, for a moment that I have filled the entire infinite universe with (magic) mud. This mud glows, and because its magic mud, it doesn't change the orbits of the Moon, Earth or anything else in the universe. But if I hold up my my hand in front of my face I can't see it, because of the mud.
Now then, at one moment all the mud vanishes, everywhere, throughout the whole universe at the same time. One second later, if I hold my hand up I can see it. But if I look in the sky I can't see the moon. The moon is still hidden in mud. In fact if I look at the sky, all I see is mud at 1 light-second (300,000km) distant. And because it is glowing mud, the sky appears to glow. It looks like I am in a middle of sphere with radius 300000, surrounded by glowing mud. But this is an effect of the fact that light travels at 300000km/s
After 1.2 seconds, the moon becomes visible, after about 8 minutes, the sun becomes visible too. But it takes 4½ years for the first stars to be visible. At this point it would look as if I'm in the middle of sphere with radius 4.5 light years surrounded by mud.
Now what really happened is that 13.5 billion years ago the whole infinite universe was filled with hot gas that blocked light. As this cooled this became transparent. The whole universe became transparent at the same time, and while it wasn't instantaneous, it happened pretty quickly. So what we see now is that we seem to be in the middle of a sphere of hot gas 13.5 billion light-years across.
Except that in those 13.5 billion years, space has stretched. This has stretched the sphere from 13.5 billion light-years to 45 billion light years, and it has stretched the light from that hot gas, so it appears to be very cold.
The photons that we see in the CMB now did not come from the hot gas that once filled this part of the universe, but from hot gas that was in part of the universe that is now 45 billion light years away. That being so, The photons that we saw last year came from gas that was in part of the universe that is now 44,999,999,999 light years away. Since we can't detect structures as small as few light years at that distance, the CMB appears as a constant even pattern in the sky, changing very little if at all.