I think I'm comfortable, but not in love. How can I work through this with losing the relationship?

I've been dating my girlfriend for 5 months now. The beginning of our relationship was fantastic. I really enjoyed her company, the sex was great, I always felt happy to see her, and she felt the same way. Now, I'm not sure that I love her and it kills me. I don't think I would feel sad if I lost her, but I know it would bother me. I care about her a great deal and I still like to see her and spend time with her. We have sex roughly once a week because I'm a full-time student and employee so I'm often tired. However when we do, it's good. I've often heard that being comfortable with someone means feeling indifferent about them, which I don't. I enjoy doing things for her and with her (I spoil her too much), we laugh together, I worry about how she's doing, and to be quite frank, she's a perfect match for me. I'll tell her I love her or she'll tell me she loves me and in that moment I mean it. I wholeheartedly mean it, but then later I find myself wondering if I actually do. At first I thought it was just a phase I'm feeling, but I'm worried that if I don't try to figure this out now, it will end up being horrible for both of us. A part of me wants to talk with her about how I'm feeling, but I'm unsure of how to explain this to her without her feeling devastated. I guess I love her, but I'm not in love with her and I feel like that's really shitty of me and I hate that I feel that way. I've casually dated a great deal and have had a few serious girlfriends (at least I considered them serious), but I've never dated anyone longer than her and maybe I feel this way because I was so used to seeing someone new every few months. Maybe it's because all of my friends live a distance away from me so I see her almost every other day (she lives less than 2 minutes from me). I don't know. I feel like I'm answering some of my own questions. I'm not sure of what to do anymore. I'm happy to offer other information if that would be helpful.
I think I'm comfortable, but not in love. How can I work through this with losing the relationship?
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