If your meatloaf is falling apart when you slice it it means you either didn't use a binding agent or you didn't use enough of the binding agent.
Meatloaf requires a binding agent such as eggs or mayonnaise and breadcrumbs or crackers to help the meatloaf bind together.
When the meatloaf binds together properly it should not fall apart when you slice it.
So if your meatloaf fell apart when you sliced it you need to either put more binding agents such as more eggs or mayonnaise in it as well as crackers or breadcrumbs.
Then the meatloaf should not fall apart when you slice it.