Plastic tends to get brittle over time.

In general but not knowing your specific application. To get started on a large snap together object, try to gradually wedge something small under a row of tabs rather than just unsnapping one at a time. The one at a time generally works once parts are most of the way apart. Think about how it snaps together and work it backwards from there.

Most importantly, use the bathroom before working on something like this. The " I just want to get this apart " can creep up on you without realizing it causing the project to be rushed. ( Same goes for pulling glass and such. )