What about a recumbent trike? Many of them fold up to a manageable size - still bulkier than a regular bike but much smaller than an adult trike. Also, if it is of any concern to you, it's not going to draw attention to you like a full adult trike or training wheels on regular bike. I wouldn't worry too much about your weight, which would be spread over three smaller (so typically more resilient) wheels.
I bought a set of Wald training wheels from Amazon for my daughter's 20" wheeled bike and I think they had some for 26" wheels as well. They were very well made. The issue you'll run into with those is insufficient axle length on most bikes to actually use them.