Until airlines realize and acknowledge that their seat standards are at least 30 years outdated, all of us who carry a bit more weight than the average person have to stress over flying. When you add that to the “traditional” stress that many people experience when flying, it becomes too much. Luckily, there are quite a few things that you can do if you are fat and wish to fly without becoming a nervous wreck.
One of the things you can do and which is actually much more effective than you would imagine is wearing a body-positive shirt. It will both make you feel empowered and it will also let people know that you do not care if they have snide comments to share. For added effect, wear one of those shirts that basically tell everyone they can f*** off.
Another great idea is to wait until everyone (or almost everyone) has boarded and then check if there are isle seats still available. If you see a seat that has been left unfilled, ask the flight attendant if they would mind you sitting there. More often than not, they will have absolutely no problem with that.
Seat belt extenders are life-savers for fat flyers. Simply put, many airlines still use seat belts that do not suffice for fat people but they all do have seat belt extenders that will make your trip much more enjoyable. If you happen to fly a lot, you might even think about investing in a seat belt extender of your own.
Finally, acknowledge the situation. The person next to you is aware that you are taking up a bit more space than the average-sized person would and ignoring this fact can only lead to awkwardness. You can do a little joke or something, letting the person know that you understand the situation but that there is nothing you can do about it.