needing advice.

So my s/o and I have been together for almost a year and a half and we haven't spent time away from each mother maybe fore a day at the max. I don't have many friends except for one and she had a couple but they stopped talking to her. Anyways that being said all of a sudden theirs this girl that I already didn't like starts to message her, she did ask me if she could and I said yes. But it's been a couple weeks and I can't stand it I don't know what it is about her I could handle the rest of her friends that she did have, but not her?? Ik a little but about her and ik know what they talk about and my s/o is completely loyal and I trust her. I just don't like this girl. So I was just wondering I've brought it up to her and she's kinda toned down the texting even tho it's still 24/7 what I should do. It gives me so much anxiety it's draining.