As relevant background, I'm a 20 year old trans man 2 months into my first ever Real Relationship. I've had zero hands-on experience with sex and dating before now, unless reading a bunch of theory (and porn) counts. My partner is nonbinary/genderfluid and has only had one (horrible and abusive in ways I won't go into here) relationship before me, though they're a little older and a good deal more sexually experienced.

We started playing this admittedly dumb and typo-ridden Couple's Sex App, and the subject of collars came up. I half-jokingly suggested using the one we already owned. (Gift from a... very sex-positive friend.) Naturally, the conversation took a turn for the less joking, and now we're incorporating something new into our Valentine's plans. I'm excited, but maybe more than a little nervous.

Question 1: What are some things you can do with a collar and leash? We've been sexting a bit, and they said they wanted an idea of what I had in store for them. I have been absolutely winging it so far -- some rough oral/deepthroating before moving into doggystyle, but I was trying to think of things that more incorporate the leash and collar. I just want to be able to give them the best possible experience I can, which leads into the next point...

Question 2: How can I pull this off without my debilitating neuroses getting in the way? Though I'm fairly naturally dominant in and out of bed, this would be the second time we tried D/s, and sparing everyone the details, the first time didn't go very well. One issue is the gulf in experience -- not just between myself and my partner, but myself and their previous partner, a much older, more attractive, wealthier, more domineering "alpha male" type of asshole. It's also why I mention the fact that I'm trans at the start of the post; my partner is exclusively attracted to men, and their ex is cisgender. It's hard not to feel insecure following up on that, even though 1. I believe my partner when they say that they wouldn't go back to that relationship even if they could and 2. Not all of their sexual experiences with this man were necessarily positive. At the same time, I can't help but draw comparisons, or even feel like I'm in some fucked up, one-sided competition a man who doesn't even know I exist, and it comes out at the most inopportune of times. I feel like I have to provide for my partner better, and love them better, and fuck them better, and fulfill them more than this person ever could, but my mind will go to his obscene wealth, or his handsome face, or his undoubtedly massive (natural) cock and all these other things he has that I never will short of some miracle breakthrough in modern medicine -- and I'm pretty sure that's not healthy, and I don't want that to get in the way of what I have with my partner. I don't think I've hated a person as much as I hate this man for what he put my partner through, and I don't want my own failings to stand in the way between them and their own healing process.

tl;dr: What can you do with a leash and collar? Also, how do I stop being jealous of the looming specter of my partner's abusive ex?