Flying is potentially a very humliating experience now for us very overweight. It's only getting worse w/ it being perfectly acceptable to be prejudice against people for thie looks if it comes to their weight.
I was about 280 last I flew but more of a 26/28, carrying the most in my hips and rear. I bring my own extender w/ me though (though I hear they are no longer allowing this??? if they catch you anyway) I've only actually needed it once on one of those tiny, tiny puddle jump planes. Anyway, I fit fine and used my extender for comfort though I dind't actually need to, I shoehorned myself into the belt they had. Anyway, I didn't have to pay for an extra seat and I fit well within one w/ no "spill-over" or
I would pay for another if I was going to impact someone else's comfort. I also am sure to pay extra for the isle or window so I can lean away from someone that's next to me should it not be a family member.
Now, that said. Here are some caveats that are making things all the harder.
1: even if you pay an upcharge for more space (some airlines won't make you pay a full fare for that "extra seat"), if they oversell the flight which they do far more often than not now, you just lost that extra space and your money. Though I'm sure you could argue after the fact to maybe get it refunded.
2: unless things have changed in the last 5 years since I checked last, even if you pay extra for that seat, even full fare, that seat isn't obligated to be next to you!! I know!!! OBSURD!

Even IF you pay that extra to assign yourself seats, for example, that seat you paid extra for to be next to you, you still aren't guaranteed that either.
My aunt and her family just traveled to Cancun a few months back. They are all average weight. She paid for each member of her family (2 adults and 2 children) on United to be seated together. Not only did they still seat tham all apart, they did it on BOTH the flight down and return flight!!! (I'm sure she probably got her $ back after the trip and complaining which requires more time and energy to rectify their problem... but that's not the point)
I personally don't like to see "big government", but when commons sense and decency is lost in business, I think they need to step in and stop poor and unethical business practices like separating young children from parents (even special needs kids!!! it's been done) and the possibility of charging by weight now. On that last issue, I really want to see actual numbers on exactly how much 100 pounds (for example) costs in jet fuel before they start weighing me at the screening or gate. And should that come to it anyway, I will stop flying and I'll drive (domestically). Period. Don't look up the articles on passengers paying based on weight and even if you do, don't read the commentary, it's vicious and unimaginably cruel.
Good luck OP! There are certain carriers that are known for humiliating passengers about their weight, so beware of them, but, there are always possibilities that any carrier can do it and you have no recourse. It's frightening.