I feel your pain. I tried getting by for the longest time with just my main character, but at some point I had bite the bullet and started paying for extra mules to carry my stuff (some people prefer paying for wardrobes, but you get more space and freedom to store different things with a mule). Once you can send things through the delivery box, that limbo where things get sent until your mules can pick them up, will itself be a form of storage (you can keep over 100 items in that limbo space). Mules also give you access to 7 more auction house spaces, which can also serve as a form of storage at need.

But if you want to stay with just your main, make sure to check that you’ve stored every thing you can on your porter slips (also keep checking after updates if new things can be added to the slips). There are also storage NPCs in the main towns that will store some items that the porter moogles won’t (usually furnishings and such, but they will hold the dinner jacket/pants items that you can pick up from the anniversary free chest right now, for example). As a last resort, while you’re sorting things around, there is the option to throw away your items temporarily, and then pick them back up from the “recycle bin” when you’re done needing the room. This is very risky if you forget to take your stuff out of the bin when you zone or log off, as you cannot recover them, so only dump things that you wouldn’t mind losing if that happens.