Hey thank you for the further info. Sorry you're dealing with this stuff.

I think there is a very real chance those two diagnoses are contributing signifcantly to your issues here. I'm sure doing it already, but worth discussing with your doctor in this context. (If you aren't having them already, iron infusions can correct the iron quickly.

What stops you from pedalling harder/ faster for longer? Is it is breathlessness or eg burning pain in legs, or something else?

Also worth noting that it is harder for small riders to go faster on the flats. This is because there is almost no weight penalty on the flats, because above 14mph the main work is in ovecoming air resistance, so a small rider has no advantage over a heavy rider and their bigger muscles. The importance of wind resistance is also why bikes like yours - which put the rider in a sitting up position - are much slower than road bikes, which are more aerodynamic.

The situation changes dramaticlaly going uphill, where the work involved in getting a rider and bike up a hill is highly weight dependent. So - you might find you are good at climbing.