As someone who is a part of BDSM community and lives a D/s relationship with my boyfriend, I"ve talked to quite a few people about spanking and how they feel about it.

It could have to do with the memories of your mom, or if you prefer to be in a Dom role it could be that is why you dislike it, many Dom's don't like spanking or any kind of rough play without them being in control, they don't like how helpless it feels or it humilates them. It would be a great idea to talk it over with your T, T's are there to help and your T may have some insight to what is causing your anxiety and confusion over it

Also as a submissive, sometimes I find the actual pain is a turn on, or the idea I am submitting to my boyfriend who likes to do spankings is what does it for me. Spanking doesn't have to be a punishment and you're right, to do those things requires a lot of trust and talking about limits and feeling safe with someone. It is really great you recognize those things

50 Shades of Grey is not a great way to learn more about the BDSM world, it is a erotic novel and a fantasy, although in some parts it does do a lot of good showcasing the emotional sides and the power there is behind D/s and different fetish plays. There are still many online communities that do a better job, if you would like to know a few feel free to PM me or PM me with any questions, It is a fun read though and I read many online stories like that when I was first exploring BDSM as well, they are a great aide in showcasing some parts of BDSM. I also found myself relating a lot to Ana in the book, because I was very confused by my submissive nature at first as well, as well as a virgin when I met my now boyfriend and Dom

I hope I was some help!