I've sprained both my ankles really badly which stretched out the ligaments.... dunno if this is similar what you're going through? My ankle was super-stiff too, and at first i just rode stirrup-less. Doing lots of walking/running helped me, as well as exercises from the physical therapist. The guy actually suggested strengthening the hips, but this was because of my sprain, so i dunno if that pertains to you.
Running cross-country with an ankle brace weirdly helped the most, i think the terrain change helped build muscles back and loosen the ankle. At first you should take it easy, but later you need to work out your ankle! do lots of stretches too. Hope this helps, and good luck!