I think the first priority is to get your thyroid hormones to an optimum. It has probably taken years for hormones to gradually reduce and we get diagnosed, so it will take a while to gradually build up to an optimum.
You will have to be patient as there's no quick-fix. Thyroid hormones (levothyroxine converts to T3) particularly T3 which is the only active thyroid hormone and it is needed in our billions of receptor cells - I suppose you could look upon T3 as the 'energy' for the cells and just like a battery for your radio it wont function properly if running out of power. When we exercise before on optimum it reduces the T3 in our cells and we don't feel so good.
You can do less strenuous exercises in the meantime till you can exercise with no ill-effects.
I have looked at a previous post and you became hypo after having your baby and that happens frequently unfortunately.
Also make sure everything is optimum. T4, T3, Free T4 and free T3 and I don't know if you have antibodies - if you have them going gluten-free can help reduce them. B12, Vit D, iron, ferritin and folate also have to be optimum.
I am sorry you are getting frustrated at present.