Can you post several photos?

You , or someone else bought the wrong dimensions for the available recess?

You should have selected a wall mount?

The only answer to the exposed sides would be to remove and see if you can deepen the recess, or paint to match surrounding wall, or ADD A TRIM, to cover the exposed sides........or buy a new cabinet that fits: ) the dimensions.

Probably the cheapest option is to add trim like Jan Moyer suggested. Otherwise, will probably have to remove, deepen the wall recess or buy another type of cabinet.