Hello @Plussizemum. I fly every three months or so with Ryanair and I'm 5'6 and 24 stone and carry my weight around my belly.
I fit in the seat fine with room to shuffle around if needs be and don't spill onto other seats. I do, however, need a belt extender. The seatbelts on Ryanair come around 5 inches too short for me.
The staff are always absolutely fine and discreet when handing me an extender (I ask as I get on the plane, rather than when i'm seated), and they do this day in, day out so don't bat an eyelid.
I do always pay to choose my seat in advance so I can choose a window seat (avoid the exit rows if you do need an extender, they'll ask you to move) and I also pay for priority boarding and try to be at the front of the queue at the boarding gate so I can settled and buckled up in my seat before any of my neighbour passengers come and sit down.
Everyone I have sat next to have always been absolutely fine, some even really lovely. Are you going with anyone? I always fly solo. If you're flying with someone you can have the armrest up.
Please don't worry about your flight. I used to do before I started flying regularly and now I actually absolutely love flying!
Some posters might also advise you buy an extra seat, but honestly, you won't need one.
Who are you flying with? Ryanair tend to be the worst for seat width and belt length, but I have always had a great flight. Word of caution though, I can never quite get the table down as my belly is in the way. I used to be terrified of using the plane toilet as I have heard they are tiny so end up for a mad dash for the loo at the airport i've landed in, but last time my leg was painful (torn tendons) and I needed to stretch it as well as use the toilet, so ended up in the plane loo and guess what, it was again absolutely fine! What scared me on that one was seeing a YouTube video of a woman of my size not being able to move properly in the aeroplane bathroom. Meh, I had room to move!
Have a great flight...there's a whole world out there!