How can I organize a really small room with a slanted wall?

Buy the plastic clothes bags that you can take the air out of, divide your clothes to spring, summer, autumn and fall, only hang the clothes for that season, the other garments are so thin you can store them under your bed.

I get having a lot of clothes. Even though my house is small, all closets have my clothes in them. Are you relegated to just one room? Can you store out of season clothes in space bags in another room or garage?

Under bed storage is great for off-season or seldom worn clothing.

The problem with assuming 'under the bed storage' is that not all beds have that option--some are solid to the floor. I'd try changing the layout of the bed first, then looking online for precisely your style of very small rooms and how 'built-ins' can give you the most space for storage, even over and around your bed. See if there's any possibility of using the space into the slanted wall? Even 3" would buy you extra floor space. Most clothes can be folded and stacked rather than hung so I'd try for upright wood 2x4's from a base going up to meet the slanted ceiling--build a framework to support solid shelves OR slide-in bins where you can fold your clothes and/or store them the 'marie kondo' way (google how she does it). Then all you'd have to hang are a few hooks on the wall for coats and jackets: a nice 12" wide finished-edge board can be run from floor to ceiling with hooks attached all the way up to hold those plus belts, purses, backpacks, etc. - but make sure the board is solidly screwed into studs to handle the weight. First I'd measure the room, measure your bed, tank, etc. take those measurements down to a Home Depot or Lowe's--see if someone there will help you plan your layout.