First off, thanks @Olo for pointing out that this is a safe space to share these sorts of thoughts!
I’m a tiny lady, so I don’t struggle with the same exact feelings as @Giant-me but I certainly can relate with somethings.
I can’t tell you where my non-con fantasies came from, but it’s something I’ve always enjoyed. A revenge fantasy can be fun - I certainly love playing a brat/bitch and being put in my place! But this is not my personal motivation. It’s totally fine if that’s what you fantasize about though, as long as it’s not affecting your IRL behavior. I know that’s easy to say, as traditionally I’m the receiver of the abuse, but I’m a very ambitious, strong willed, feminist woman. It doesn’t feel good morally to have these fantasies…that part I get.
I’m not sure if your fantasies played a role in your relationships ending, but it at least seems like there was some lack of clear communication. Maybe not sharing enough of your fantasies kept things disconnected or boring in the bedroom. But that’s speculation. The clear fact is that she didn’t feel comfortable enough to share her fantasies with you, because instead of communicating, she turned to another man. Most people in the community under share - which I get. Nobody likes to be vulnerable. She might have had the most boring, vanilla fantasy in the world, and still might feel as embarrassed to talk about sex like the rest of us kinky weirdos lol. I highly recommend in any future relationship, to prioritize transparent communication and unbiased acceptance. There are people in the community who communicate their fantasies, find out they’re not compatible, and they break up. It sucks. But every couple I’ve known like that has moved on to find better, mature, stronger relationships.
The way that I came to terms with my dark fantasies was sharing them with my husband. Not just verbally, but actually engaging with him in the fantasy. Together. My husband has always enjoyed being a smart-ass bully with me, and enjoyed a few dominant non-con fantasies here and there (slapping, choking, free-use, etc), but my size fantasies, and generally my dark fantasies, went quite a bit further than what he was interested in. There are some fantasies of his that I humor, and vice versa. With this mindset, we were able to explore and experience lots of fun, sexy new things we might not have experience otherwise. We’ve found over time that we actually mesh pretty well sexually. While many of our engagements are non-con, my husband is in no way a monster. He is a romantic, thoughtful gentleman!! 
Why? Well, he might be waking me up in the morning as he pulls down my underwear and covers my mouth with his hand. But he (1) knows I love it (2) woke me up around the time my alarm goes off so I don’t lose sleep (3) considered if I had work/appointment to go to wherein he might be taking up precious time (4) considered if I was feeling well the night before, am on my period, etc. He’s actually pretty thoughtful!! I live a Dominant/submissive relationship with him, and those provide us certain pre-agreed to rules/limitations for each of us to live by. If he walked up to me right now, held my arms back and kicked my legs open to fuck me, this would be a POSITIVE interaction for me. It’s all about personal preference, there’s nothing inherently bad about it. What you do in your private home with your consenting partner is your business.
When my husband and I are roleplaying, I revel in him treating me like an object or pet for him to do as he pleases. He can choke me out, leave bruises, take a piss right in my face while he laughs, and I love it!! Doing something that’s subjectively bad, doesn’t mean my husband is a bad man! Everything he does is actually with CONSIDERATION and RESPECT for me!! My husband has supported me as a friend and partner, whether that be starting my own business or running my own convention, he’s never held me back from any ambition. He is cool, calm and collected, never loses his temper and always makes time for me. He always considers my opinions, cares about my thoughts and picks me always above anyone else (his family, friends, etc). I also return the respect to him as well. That’s the beauty of communication. Security. Validation. The maturing of our relationship was worth every single uncomfortable conversation.
And I mean, I had a conversation with him once where I wanted him to facesit on me, and I wanted him to twitch his asshole, because it was scarier for me. Like, who has these conversations? Apparently, me.