Looking for guidance on relationships

I’ll try to keep this straightforward. I'm a 25-year-old male in a two-year relationship with a 25-year-old woman. We've been engaged for about ten months. Lately, I’ve been feeling down, depressed, and generally unhappy, which has affected my sex drive. I left the military about a year ago and relocated for a new job, which my fiancée and I did together. We've been living together for most of our relationship, and overall, our dynamic is strong—we communicate well, share numerous interests, divide household tasks fairly, and rarely have conflicts. When we got engaged, I thought it was what I wanted; it may have come a bit sooner than I expected, but I was on board. As time has gone on, though, we haven’t made any wedding plans, and when the topic arises, I find myself lacking the enthusiasm to move forward. I can’t quite explain it—my mind says yes, but my heart isn’t ready. I love her deeply; she’s been my support and does so much for me. After discussing my feelings with her, I feel like I’ve completely shattered everything. I expressed my unhappiness, not knowing why, and stated that I’m not ready for marriage at this moment. Since then, I’ve moved to the basement for some space to think things through. I genuinely don’t want to hurt her, and this situation is tearing me apart. I don’t want to imagine life without her, but I’m uncertain about how to proceed. My indecision is overwhelming, and I’m starting to regret everything. I can’t pinpoint the reason I’m struggling to commit. We have a solid financial situation, good jobs, stable incomes, and virtually no conflicts. So how do I work through this?