Bill sometimes wished she could tell her younger self that the adverts weren't cheesy and stupid, it really would get better. Being black and gay would never be easy, obviously, but she would see mind-blowing things and she would help to save the world and, honestly, it really does get better.
Even though she'd almost messed it all up and given the world to a bunch of dead monk creatures, it hadn't been the end. She'd fixed it. That would be a thing she could do.
Bill wouldn't tell her younger self that all the world saving stuff made having a date with the girl of your dreams complicated and prone to interruption. And she definitely wouldn't mention that sometimes the girl of your dreams turned into a spaceship and flew away.
(She asked the Doctor once about visiting her younger self and he'd given her a two hour lecture about time and paradoxes and theories about time stability. She hadn't understood all the maths he'd scrawled on the blackboard, but she'd followed enough of it to work out that meeting her previous self was a Very Bad Idea. Capital letters totally needed for emphasis. Giving the Earth to a bunch of undead monks would look like a minor blip compared to the bad shit that would happen if she met herself. She never asked again.)
(She was determined to send a letter, though. Just a letter. That couldn't destabilise the whole of time and space, right? Except the Doctor had been really careful never to take her to a time period when sending herself a letter might be possible. She suspected he'd worked out her cunning plan.)
Bill had thought nothing would scrub away the memories of living under the monks' rule. It was six months, right? Six months of being afraid and running and, oh god, the loneliest six months she'd ever known. Except those months time were already starting to feel hazy and dreamlike, and she had to really work to remember all the details. The Doctor said that was normal.
Bill thought normal was probably good for most people, but she needed to remember. She needed the details seared into her brain, in case she ever had to make that kind of choice again.
(Bill would probably make the same choice if she got a do-over, if she was honest. But she liked to pretend she wouldn't.)
On the list of cool and amazing things she'd seen that she couldn't tell her younger self about, Bill decided that Victorians on Mars ranked pretty high. I mean, Victorians! On Mars! With really Victorian space suits with ear trumpets! How cool was that?
Okay, yeah, Victorians came with Victorian attitudes, even on other planets, but they'd been open-minded and they'd learned. And the Empress killed the ones who didn't.
Bill felt a bit guilty about that thought. She was starting to understand why the Doctor didn't fall apart every time he saw someone die, and she wasn't sure she liked that. She didn't want to become the kind of person who saw a body and went "oh, well, I get why that happened and I'm not going to stop it".
That seemed like the start of a slippery slope that ended in megalomania and mass graves and she'd met creatures at the bottom of that slope, she knew what she was talking about. So when she caught herself feeling slightly gleeful at Catchlove's demise, she gave herself a stern internal talking to.
It was still hard not to be excited about the idea of Victorians on Mars. And Ice Warriors! Led by an Empress! Bill had barely restrained herself from punching the air at that. Matriarchal warrior societies for the win.
(Was it matriarchal if there was only one woman? Bill made a mental note to ask someone in anthropology the next time she was near their faculty building.)
The Victorians on Mars thing was all going swimmingly right up until they walked into the TARDIS. Bill didn't know what the history between the Doctor and Missy was, but she didn't need the details to know it was heavy and bad.
(Possibly also sexual, but Nardole got weird and twittery when she suggested that, so she didn't say it again.)
She stared between them, trying to work out what they were saying with the long looks and the subtext, but she couldn't grasp it all. There was too much. But the happy glow from fixing the Martian issues faded away and she had a new thought.
If she ever got to send that letter to her younger self, she would tell herself that things really do get better, but they also get complicated and, sometimes, they get scary.