Living with a new flatmate

Hi everyone, this is my first post looking for advice or just similar experiences.

Since I finished university three years ago, I have lived with the same person (let's call them R) - we had other flatmates too through the years, but I have always had this one person who I'm very close to. We were really good at living together and we've done it across two cities at opposite ends of the country, until recently. This summer they moved in with their partner, and I now have a new flatmate without R also being in the household for the first time in my renting life.

This is obviously a big change, I still hang out with R regularly though. It's been about six weeks since we moved. I have really been struggling with my new flatmate. She is a friend, going back around a year, and needed somewhere to live at the same time I needed to move, so it worked out for us. We are both trans, and she also is autistic and has ADHD.

I just find myself very easily irritated by her for no real reason - like a knee-jerk response. I am coming to the conclusion that maybe the amount of close contact living with someone requires just means that almost anyone in the world would annoy me as a flatmate. R was just a lucky exception and I was somewhat spoiled by living with them for several years.

There are some things that can be addressed - this is her first time renting and she definitely has some stuff to learn about taking care of a space. However I find myself weirdly irritated by things that seem "studenty" (she's four years younger than me, as I said has never rented before now, and is a student) or just random mannerisms that bug me that feel unreasonable. There's no nice way of telling someone you just find them annoying, even if some of it can be chalked up to a difference in life stage. I also wonder how reasonable it is to even justify being annoyed at things by ascribing it to "student behaviour".

This makes me feel like a bit of a mean-spirited person which I don't love. I am making longer term plans to hopefully be able to get my own place to live on my own and stop renting, but this won't happen for at least a year. I need to learn to cope as I've been really struggling with depression over my living situation.

An example of something I would like to deal with better is that previously when living with R they would have someone over and it would be fine - maybe they'd watch a film in the living room and I'd hang out in my room. It felt very much like I still had space. When my currently flatmate has someone over it feels very much like my usable space in the flat has shrunk to just my room, which feels like a huge double standard. Maybe it's just because this flat is a bit smaller, or because I like R and the people they get on with more, or maybe R was less loud, but that last one doesn't feel true, and I have noise cancelling headphones.

I was wondering if anyone had any tips to adjusting to coping with new flatmates and sharing a flat with people you have less in common with. I'm not forcing myself to socialise or anything, I mostly keep to my own room and don't interact much.

These emotions feel very irrational on my part and very ungenerous, and I'd like to be more patient and tolerant of others around me. I think I was spoiled living with someone so compatible with me as R for several years, and I need to build my tolerances back up.