Hi Nursanney

I carry my weight in my hips (well, a lot of it) and in my thighs. I have flown and I do fit in the seats, but I am very self-conscious of it. I am "snug" but I've seen others who are larger than me (in the rear) who have flown, so I know they CAN fly.
I suggest you call the airlines directly and tell them what type of plane you will be flying on and ask about the seat size (maybe you can ask about the size of the seats/then draw on a paper the size width of the seat and see for yourself how you will fit). Just a suggestion...
There may be some kind of "consumer relations department that addresses these types of concern for us larger people. A friend of mine called Disney World before going because she was concerned about riding rides with her child and not fitting into the seats-they had advice and diminsions...it may be that the airlines have that, too.
There may be more room in the seats that are closer to the front-I don't know, but I'm thinking of the ones that face the wall/where there aren't any in front of you. That may give you more room. At least you won't have to deal with the seats in front of you, too.
You may want to try an aisle seat, too. I would think that would be helpful in giving more room.
Good luck! I hope this helps!