Best way for you to figure out what character to play is to determine the archetype of what character you want to play. Some examples of archetypes include Rushdown (characters who rely on aggressive playstyles, although they can sometimes lack a projectile), and Zoners (characters who rely on projectile play to pressure opponents). Knowing what kind of playstyle you enjoy playing will help narrow the cast down, although it's still okay if you're not really sure. Honestly it wouldn't hurt to experiment with all of the characters in training mode to get a feel of each character, since each character can offer different tools that might appeal to you.
Once you start experimenting with each character, there are characters who are bound to appeal to your liking. You don't necessarily have to go out and disregard your previous characters that you thought were nice to play, so keep those characters in mind. Experimenting with each character can vary depending if you are a competitive player or a casual player, so results will vary. Competitive players will know a lot more information about their character than casual players, such as frame data, out of shield options, combo setups, etc. A casual player will solely choose a character that appeals to them because they enjoy the feel of a character, but they don't take into consideration the frame data, and other stuff regarding their character. Depending on where you stand, you might be the one checking a character's combo options, or just seeing which character feels nice to play without regarding competitive stuff.
Once you're done and you have some characters in mind, you can place them onto your secondaries list since you might change your mind and end up choosing a different character. Secondaries are for characters where you know the idea of how to play them, but not with extensive knowledge like you possess with your mains. Eventually with enough experimentation, you are bound to have extensive knowledge on a character/characters more than others. If I were you, I would keep your mains at a low number (1-3), since having multiple mains would mean that you would have to put the same effort of practicing combos and other stuff into each character, and that can sometimes be difficult to do. If you're good at keeping track of where your flaws are with each character, maining three characters would not be an issue for you.
