TL;DR:
I (M, 28) have been close friends with her (F, 26) for years, but she's in a relationship. Over the past 8 months, we've been texting daily and spending a lot of time together. Recently, I went out with a single female friend, and her behavior changed—she texts less, seems distant, and occasionally gives cold replies. She still spends time with me but acts differently. I care about her (and like her) but don’t want to stop seeing my friend or lie to her. Looking for advice on how to handle this situation.

Hello everyone,
I (M, 28) and she (F, 26) have been friends for several years. She is in a relationship.
Over the past year, due to a shared hobby, she started texting me frequently until it became an everyday thing, from morning to evening.
This has been going on for about 8 months now.

There have been moments when she seemed confused about her feelings for me—whether I was something more than a friend or just her best friend.
In any case, we’ve always managed to work through any issues, and she’s still in a relationship. However, during the week, I see her quite often, at least 3 to 5 times, on top of us texting daily. She would even come over to my place often, staying from morning until late at night.

Last week, I went out with a female friend of mine—a person she initially got along with but doesn’t anymore (I’m not sure why). I should add that as long as I was hanging out with other female friends who were either in relationships or not interested in me, she was always calm. But this time, when she found out I was with this friend “X,” who is single and doesn’t seem to be interested in any specific guy, something between us changed.

She started texting me less, especially in the mornings, when she used to be the one to text first. Now, she rarely does, and when she does, it’s usually around lunchtime.
She hasn’t come over to my place, claiming she’s been super busy and has lots to do.

Last week, when we were together, knowing I had a birthday party to attend later that evening where “X” might be present, she said something like: “So who’s going tonight? Excluding X.”
I replied by telling her who was going, and of course, “X” wasn’t there. I found the way she phrased the question a bit odd.

But what’s even stranger is her behavior. When she’s with her boyfriend or goes to his place, I can’t say anything. Yet, if I spend time alone with another woman, she suddenly distances herself.
She still finds time for me, invites me to go out, and suggests doing things together, but I’ve noticed she has stopped initiating texts as much. Sometimes, her replies feel cold or distant. At times, she even leaves my messages on “read” for hours before resuming the conversation—or doesn’t reply at all.

Any advice? Opinions? I’m open to hearing anything.
I’d like to work things out with her because I care a lot about her (and yes, I like her), but I definitely don’t want to stop seeing “X” or have to lie to her just to keep her close.

What I should do?

Thanks in advance!