Something Wrong with RJ45 Connector

Good day, everyone! Please excuse my naive mistakes, but could someone please explain it to me? My network cable will not fit in the RJ45 CAT6 connector even though I have a CAT6 cable and connectors. Is there something I'm missing? I was under the impression that a CAT6 cable could be terminated with an RJ45 connector in the same way that a CAT5 cable could... CAT6 tiny cables, on the other hand, appear to be substantially wider than CAT5 small cables.

Not really within the remit of this forum but it sounds like you are mixing RJ45 with the RJ11 connectors used in router/modem to phone cables

cat 5 and cat 6 standard clear plugs will generally work with each other, assuming you are using normal solid core, twisted cable? But there is variations to those as well, solid, stranded and universal.
Under ground cable (this is gel filled to keep water out) cat 6 also won't fit into a normal plug.
However the sockets are different.
hope this helps?