Afraid of getting married

The title says it all. I'll keep this brief, but there's a lot to unpack. I’m a 25-year-old male, and my fiancé/girlfriend/ex, who is also 25, and I are going through a tough time. We've been together for just over two years, during which we've lived together for most of that time. In the last year, we both left the military, moved across the country, and got engaged about nine months ago. Up until recently, our relationship was great—healthy, an active sex life, and we’re best friends. I love her deeply. However, she has been somewhat pressing about our wedding timeline, which I didn't mind until now. For the past four or five months, I’ve felt increasingly unhappy. I can’t pinpoint why; she’s perfect for me, and there’s nothing about her I dislike. Yet, I find myself feeling unfulfilled in life in general. My hobbies don’t excite me, work feels mundane, and sadly, I’ve started to view my relationship in the same light. When marriage was brought up a few months ago, I didn’t feel ready. I confided in my mom, who suggested that perhaps she isn’t the right one for me. I shared my unhappiness and this opinion with my girlfriend, which ultimately strained our relationship. While she initially tried to support me, her mood vacillates between being understanding and frustrated. Now, she has a negative view of my mom, and I can see how my mother’s opinion weighs on me. It’s like a wedge driving us apart. To give us both some space, I’ve moved in with a friend, and I’ve started therapy. I know I have a lot of responsibility for how things have turned out, but I can’t grasp why I have these mixed feelings. Even with minimal contact and not living together, every day feels overwhelming. I’m struggling deeply—occasionally not eating or sleeping, drinking too much, feeling withdrawn, and not performing well at work. Why can’t I make a decision? I worry I can't go back until I feel completely ready, but I’m uncertain how to achieve that. Have any of you ever experienced a strong, undeniable “yes” when it comes to marriage? I’ve had some rough breakups in the past, but this feels so much worse. Is this a sign that I’m head over heels in love, or am I just co-dependent? I feel trapped in a cycle where every day that passes makes me feel like she’s slipping away, which is painful knowing I’m causing her hurt. It’s a difficult time for me; at points, I feel like it would be better to not be here at all. I’m caught between wishing I had more time to figure out my feelings and not wanting to waste her time because I’m unsure. I can’t picture myself with anyone else, and the idea of her being with someone else makes me feel physically ill. I’m seeking advice—has anyone else been through something similar?