Tommy Loughran taught himself and became champ, it didn't mean that he was ignorant, it just means he had to take a different approach. But most people aren't in it to become on top, sure they want to good boxer, but they're not going to go through all the steps to get there. Is it possible to teach yourself and beat most of the fighters out there, sure, it's more difficult. I'm not sure if Loughran had sparring partners, but in those days having a lot more fights than is the case to day formed the important eduction of becoming a good fighter.

Unless you're going to approach this like a job, it probably be best to join a gym and to take it from there. The hard part is to find a gym that is on the same page as what your goals are.