In ponyplay, I retain my human persona. That is, I am a human forced to become an animal. I am forced to act like a horse, and I am treated like a horse. Since I keep my human persona, I often feel humiliation when I am degraded to animal status and will actively resist efforts to make be behave as an equine.
I know some people involved in ponyplay look down on "forced ponyplay". This is fine, we all have our wants and needs and forced ponyplay isn't for everyone (also having been in the bio-horse world for many years, the cliquishness of pony play is nothing in comparison). Frankly, I think all ponyplay is valid and you should do what makes you happy or turns you on. Although I really wish I looked incredibly hot running around in my ponyplay tack, I know I look ridiculous, but hey, I like it. Moreover, I expect knowing I look like ridiculous is part of the appeal of forced ponyplay. Anyway, I really am digressing here.
Until recently, I didn't really get into the equine role-play mind space in my ponyplay scenes. It's always been "I'm a person; you can't make me act like a horse". So part of this is getting off on the whole humiliation/degradation thing. However, I recently realized that an equally large part, for me at least, is also not being able to really let go of my inhibitions.
The best analogy I can give you is with singing or acting. If you can't sing and you know you can't sing, there is a large amount of self-consciousness that you have to get over. The same is true for ponyplay (at least for me).
I'm terribly self-conscious as a pony, and I absolutely hate having to make horsey sounds (and I hate being called "horsey"), especially the whinny. If you really want to dominate me, make me whinny. I'll endure running on a treadmill, standing forever tied to a post in hoof boots, whippings, almost anything over having to whinny. During ponyplay there is little more degrading to me than having to whinny. I don't know why, but this gets me every time. If you can get me to whinny, you've won: I've fully accepted my transition to equine.
Anyway, I've noticed that over the past few training sessions that I've been able to almost completely let go (I've been able to pinpoint the time to after my trainer hogtied me by my bit). This came as quite the surprise to me. In forced ponyplay being able to let go seems like the exact opposite of the reason for trying ponyplay.