Food blinks when you its at half duration/potency and you can choose to eat more/use another/or wait and let it run out, depending on your situation. You can simply open up your inventory to click to eat more of whatever food you want, maximum 3 at a time.

F is the default for forsaken power. Your current selected power show in the bottom left. If you have a power activated it shows up in the top right with the other buffs/debuffs.

You need to plan Forsaken power use, as the animation to trigger it takes a couple seconds where you can't do anything else, and a great way to die if you are being swarmed in combat.

Note for group play, if a player triggers a Forsaken power, all close by players get the buff as well, so you can coordinate use to get more uptime.