I think we can become obsessive about all sorts of aspects of our weight loss, including our exercise. That doesn't necessarily equate to a disorder or . Frankly I suspect that the extreme care some of us take to ensure we keep making progress, is what we will need to our weight loss. I think it's reasonable to have some fear about that. There is a genuine danger. We all know that most people do not manage maintenance long term. We are all trying to beat those odds. Mostly the sense of fear isn't obvious and is helpful. Personally I don't find my fear unreasonably intrusive and will think it's reasonable for me to become less fearful some years down the track, maybe.
If it happens that we are set up to have to chose between feeling compelled to exercise and feeling compelled to overeat, that's not necessarily a problem is it? I guess it might be if the compulsion made us repeatedly exercise despite the fact that we knew it was hurting our bodies in some way, but I haven't met anyone here who might fit into that category.
Maybe the issue that can be a problem is if we feel that there is just one way to exercise that we know we can rely on for weight loss or weight maintenance, and become anxious if we can't continue the exact program we originally relied on for our results. Of course if we manage to move past that stumbling block, we often find that having a change brings more benefits than than problems anyway.
I wonder if my comments are relevant to the issues you raised? Maybe your real concern is in your last question “Did I master my mental domain?” Perhaps you see that you were in a situation where there was a "mental domain" controlling you - compelled to eat whilst knowing it wasn't in your best interests, or even what you consciously wanted, and wonder if you are now in a situation where you feel/may feel compelled do something else (exercise), whilst knowing it isn't in your best interests or even what you consciously want? Personally I recall a massive intellectual awareness that my eating was harmful, and divided emotions. My emotions included feeling disgusted by my lack of control and its consequences, yet also feeling buoyed/brightened for short periods by satisfying the urge I had to eat badly, or by the physical pleasure of eating. I recall taking junk from my fridge to eat while having the idea that "I" was watching myself do it in wondering, horrified resignation. Perhaps that's just out and out denial - but then there can be automatic unhealthy eating that's almost unnoticed. Also, I remember the first time I kept walking without consciously deciding to and heaving a sigh of relief that the part of me which previously would sabotage my conscious plans seemed ready to sometimes help instead of hinder.
I find it helpful to see myself as having more than one mental domain. Perhaps that is similar to what you mean when you separate the idea of “I” from “mental domain”. Yet of course there was only one me doing everything I just mentioned and only one mind and it is partly composed of my conscious thoughts, partly of barely conscious memories, partly it's a register of sensations, and of course the complicated effects of hormones and other chemicals. There are such a lot of different ways we can think about what we mean by ideas like self and mind. One way I see it is to consider the less conscious part as something we can consciously train to support our conscious decisions. As for example, we do when we write about our intentions and weight loss beliefs and victories, or as you did, repeating your determination and intentions aloud. Also our repeated behaviours, are a kind of training. I guess all that training also affects what's likely to pop up in our conscious thoughts in future. If we have trained our mental domain, does that then make us its master? If we are master, does that mean it can never run across the road without permission? I guess it's less likely that it will if we keep an eye on it for signs of imminent rebelliousness and if we have useful plans that we implement when we see those signs. Also if it does lash out in rebellion, does that mean it won't come slinking back with lowered head and anxious eyes in apology? Or that we can't reassert control quickly. Surely if those things apply then mastery's still there.
Then again... You may be talking about the way that you might now believe you have done a useful, substantial amount of exercise, and eaten well, yet still feel that you ought to make use of some remaining time in your day to exercise even more. Maybe you feel uncomfortable or even guilty if you don't, because that would go against the habit you've now developed. Perhaps you have the mastery over your mental domain, you took it for a run this morning, yet it's used to going after dinner and there it sits at the door with its lead in its mouth! I'm likely to be a bit uncomfortable about that myself, but in all seriousness if that's the scenario the master still gets to choose, and doesn't the result depend on whether or not the master has other plans for his time?
) Exercise (at least the walks in the fresh air and sunshine) make me feel way better than compulsively eating.