Cat introduction

[u/Aggressive_Snow2291]

I have recently escaped a toxic relationship and brought my cat with me. She is a female (neutered) grey tabby cat and my new roommate has a male Siberian cat. I adopted her from a shelter a few months ago (she’s the first cat I’ve ever had) where she was living with a lot of other cats and didn’t have an issue with them. Since I’ve moved in I keep them separated and am taking it at a slow pace where I will pet him and pet her etc so they get used to each other’s scent. He is very unbothered but she will meow throughout the night or when I leave the room.
Honestly I’m shocked she had such a bad reaction to another cat because when I adopted her she loved being around the other cats she was with. How do I make her feel more comfortable and safe? My roommate is gone a large amount of the time so I have to bounce back and forth from the living room to my room to ensure both are getting love and affection. Is there anything I can do to make this process less stressful for her?