Photos would be great to identify what rifle it is you have and whether it is the correct bayonet for it. It is sounding like you do not have the right bayonet for the rifle.

A cavalry carbine accepts a bayonet that fits on the underside of the sight base. There are different types of bayonets for these rifles. It is permanently attached once in place but you do not need to remove the sight base. These bayonets are long spike bayonets.

If it is a type 38 that accepts the folding bayonet, there is no need to remove the sight. You mentioned a "sleeve" which seems to indicate it is a bayonet with a ring that goes over the barrel. This type folder does not go on a cavalry carbine, they go on the 38's. These bayonets are short bladed bayonets.