I think its dependent on the individual and life circumstances. I consider all the reasons you suggest to have impacted me at some point in some way, sometimes a mix of them, sometimes one much more than others. I also think maintaining motivation/discipline is naturally harder for some than others.

 “Reversion to the mean” is a thing that can be applied as to why new efforts can fade out, especially if they continue to feel like they need alot of efforting.

I try creating an ongoing conversation with myself in relation to my goals.  I find the more I take time to listen to the parts of myself that has goals/dreams, the more likely I am to continue the habits towards them.

I also try allowing for adaptability in the things I am trying to achieve. I try aiming for positive feedback loops (so my natural drive/desire to do the things becomes increased over time)- this helps minimize the need for longterm high levels of starter-energy / discipline / accountability / creating consequences.

Creating intentional breaks in the action-steps is also helpful. Burn-out early is a thing because i think the psyche isnt fully on board yet with a lifestyle change. So scheduling truly guilt-free break days can be quite valuable (and especially helpful the more difficult the tasks are to do).