One of my conditions is that I don't produce anywhere near enough cortisol naturally which is adrenal insufficiency (symptoms include: hypoglycaemia, dehydration, weight loss, and disorientation and may also include weakness, tiredness, dizziness, low blood pressure that falls further when standing (orthostatic hypotension), cardiovascular collapse, muscle aches, nausea, vomiting, and diarrhoea.) I can tick way too many of those symptoms. Luckily in my case, the cause is known/assumed to be known - I am a severe asthmatic with a very high dependency of inhaled steroids to manage my asthma, nothing else really works, so 9 sets of meds and a nebuliser (and I have several chronic complications not least of all a major dairy allergy which limits treatment options for everything and long term damage to both my throat and lungs due to constant coughing (a rare form of tracheomalacia and bronchiectasis to just add to the fun)). I also find it difficult to put muscle on.
What I am saying is that you need to know your conditions and cycle within it. It is that simple.
Set yourself realistic targets/goals and only very slowly increase them. If you are too tired that day, you have to accept it and work out how to avoid it happening too often. It's not easy - I have little social life (not that that actually bothers me) knowing I have to be in bed by 9pm otherwise the next day does not happen. I don't work, I am still working on working out how to do that but I am fit enough to stay out of hospital and can manage my conditions myself without too much intervention or needing to visit the hospital or Dr's - somewhere I prefer to avoid because I always come back with a bug just from being around ill people. My lungs and immune system just don't cope well with the dr's surgery or hospitals!
For me managing my conditions means forgetting how quickly or otherwise I'm going (which is far more likely), forgetting "racing", not caring about people overtaking me, just getting there at a speed I can manage and know I will be able to function when I arrive at the other end. I now do a 22 mile (domestic) commute each way 2-3 times a week now, but it took me a while to build up to that. I am fortunate to have the support of both my severe asthma consultant and one of my GP's. My endocrinologist is less keen on my cycling but acknowledges that my other health issues would hospitalise me permanently if I didn't stay fit. Roughly 1 day in 7 is a write off for me, and my endocrinologist told me last week that that is actually very good. I am still trying to get my head around that!
My conditions also mean I don't 'travel light'. I always have to have food available, including something with high sugar and something that can deal with adrenal crisis - I can't always balance potassium and sodium in my blood stream, something that can be fatal very quickly. I have to take into account that I may need to get warm fast (so always have extra clothing available) and have to avoid getting soaked to the skin - good waterproofs are useful here - rather than just avoiding cycling in the rain. I also wear medic alert bracelet and have a card in my wallet so that if I am found unconscious I can be treated, but am much better at managing things now, and am not afraid to stop cycling and eat even if I am only 30 minutes into a ride, if my body tells me I need it. You will (if you have not already) need to learn to pick up on these signs. As for motivation, cycling buddies help, commitments help but for me the knowledge that I will (and have been) bed bound/house bound keeps me going.
So what motivates you normally? Do goals work, targets work, rewards? Do you have friends that can come out with you or you can arrange to go over and visit and cycle there?