My relationship (19M and 18F) is taking a toll on my mental health. What steps can I take to improve the situation?

I want to start by sharing my situation: I’m a 19-year-old guy, and I’ve been with my girlfriend, who is 18, for about four months. The first couple of months were great; we spent a lot of time together, cuddled, and talked for hours every day. However, things took a turn after those initial two months. She started being distant, with slow replies and a desire for space. I used to be somewhat cold and didn’t want anyone getting too close, but that changed when I became physically close to her. I really enjoyed holding her and being close, but now, for the past two months, she has seemed to withdraw both physically and emotionally. She tells me she’s still interested, but that just adds to my anxiety. For the last two months, I’ve been dealing with chest pain and high blood pressure that has even led me to the emergency room. I had a panic attack on New Year’s Eve while we were in another city with her best friend. I felt completely alone while they were occupied with their activities. What triggered my anxiety was overhearing them play a game I had bought her, and when a question about happiness in the relationship came up, they went silent and then laughed. That’s when I started shaking and couldn’t breathe; I had to leave the apartment in that moment. When I returned, she only asked if I was okay and didn’t offer much else. During our trip, she stayed on the other side of the bed, facing away from me every night. I’ve been feeling unwell for the past two months, and I’m at a loss about what to do. I’m afraid and don’t feel safe opening up to her anymore. She doesn’t initiate conversations but wants me to keep messaging her, and I’m struggling. I don’t recognize myself anymore. I’ve given her everything—emotionally, physically, and financially—as much as I could, and I wish she would reciprocate in some way. Instead, she always seems irritated. When I tell her I don’t feel like I’m enough for her, she reassures me that I am, but her actions don’t reflect that. Our conversations often end with a simple “fair,” and nothing changes. I know she has her own issues, and I’ve tried to be understanding, but I’ve changed so much for her that I don’t even recognize the person in the mirror anymore. My self-confidence has completely disappeared. I love her and don’t want to walk away, but this situation is becoming unmanageable for me. I’d really appreciate any advice or support, and I apologize for venting, but I can’t keep carrying this weight alone.