Healthy eating comes in time, not overnight. Always remember to be kind to yourself

Don’t forget, the ones, “I wish I was in the pool peeing now” folk. 

I’m jealous.  I’m in the same situation you were and would have to lose 100 pounds to get to where you are. 

If you are planning on issuing a chapter every week or so, then my advice would be not to release the first chapter until you have six chapters in the bank.  A lot of stories have died around here over the years because authors went with the flow, painted themselves into a corner, and couldn't find a way out.  So, start writing, and don't worry if it's stream of consciousness because its a draft, and you will be ironing out the awkward or dead end stuff in the early chapters once the arc becomes more clear in your mind. Speaking of the arc, the day care / boarding school setting has been used enough times in this forum that you should start out by asking yourself how your story is going to be different.  Give yourself a clear focus, and writing the text that gets you there becomes a lot easier.  Remember, Margaret Mitchell of GONE WITH THE WIND fame, wrote the last chapter first.  This is always sage advice ... always. 

First, I would say get used to peeing whilst standing up. That way your body will allow you to "go" much easier. Plenty of water regularly would for sure aid the need to pee. Caffeinated drinks (tea, coffee, Coca Cola) will also increase the need to go to bathroom more regularly. Allow yourself time with this, it isn't going to come straight away. But the more you avoid actually using the toilet and going in your nappy, the greater chance that you'll have an "accident". Good luck!