Slowly growing more unhappy in my relationship

My girlfriend (20f) and I (20m) have been dating two years. We met through a friend, and clicked immediately. We started dating quickly, about two months after meeting. The start of the relationship was electric, I'd never felt a connection like that in my life. I craved her attention, and she craved mine.

We were each others first. We did it multiple times a week, anywhere we could. Doing anything in public was hard, we were so touchy that we could barely get through a night out without being inappropriate.

Then after 6 months of dating, she was expressing to a friend how she wasn't happy in some areas of her life (the friend is a straight female. No romantic interest) the friend pretty promptly told her to end things with me, that I wasn't helping anything. Thankfully, my girlfriend thought things through and instead ended the friendship with her friend. She evaluated her problems and realized I wasn't the source of her unhappiness.

Even though she chose me, things have been different ever since. I was away visiting family when she had that conversation with her friend, and she told me about it a month later. Prior to that conversation, everything was as I described earlier; we couldn't live without each other. But after that convo, she became distant. She'd take naps all afternoon. She would stop responding as early as 5 PM, and the naps are usually at least two hours. Some nights she wouldn't text back, instead sending a text in the morning. She never let me know if she was taking a nap, she'd just let me wonder why she disappears all night. I let her know it bugs me, and that it's fine if she takes naps, I would just appreciate a text so that I know she isn't ignoring me. She continued without texting me.

I swept that under the rug, because everything was fine in every other area. Our sex life was great, our conversations were deep, and we could talk about anything. Then recently we'd have sex, and she'd clean herself up and immediately scroll on her phone. I thought nothing of it at first, but I noticed something that bugged me. For several months, she's been having slower response times when we text. I understand she has a life and gets busy, but why does it take her hours to text me back, but then is on her phone most of the time we spend together? It makes me question if she's actually busy all the time, or if she just doesn't want to talk to me anymore.

We've discussed marriage, and early on we were ready to rush. Recently however, I'm not so sure. When I ask to spend time with her, she doesn't seem interested. I'll say "do you want to go out Saturday?" And she'll say "mmm yeah if you want" but never seems like she WANTS to hang out. I feel like a backburner, someone she'll spend time with if it's convenient, but that she'll never really crave me like she used to.

I've discussed these concerns with her, and she'll verbally comfort me, and things stay exactly the same. I'm not sure what/if there's any advice I'm looking for, more just venting. Feel free to ask more questions, I'm willing to give more details.