Tile backer board first time

Hi all, I'm tiling my bathroom that has plywood boards on the floor. Apparently you are supposed to put backer board first, I got HARDIEBACKER 6mm. It's my first time tiling with them (and 3rd in total) so I don't know how tight they have to be around walls, pipes, toilet etc? Can you fill gaps somehow? Can you put small pieces in bigger gaps? Cutting them turned out to be more difficult than I thought, photos attached - do you think I should start from scratch?
Any help will be greatly appreciated!

Hardiebacker can be a real pain to cut which is probably what you've experienced. The Hardiebacker is there to support the tile ultimately so it needs to be sound and flat. You could fill the gaps with tile adhesive, don't forget to bed it all down on tile adhesive and tape the joints. Screw down and check continually with a level that it is flat.