You can only hold a limited amount of Mana Flux at one time; beyond that limit, it flows out of you like water from an overflowing cup.

The starting Mana Capacity for all characters is 2, allowing you to hold that many charges of Mana Flux at once. If you gather more, any excess is immediately lost, before you have a chance to use any actions to spend it.

Your Mana Capacity can be increased via the Mana Well ability granted by some Keystones, and is temporarily decreased while you wear armor with a Mana Dampening rating.