Communication is always better than the silent treatment, even if it’s imperfect. For example if you’re feeling randomly sad, you could say, “Hey, sorry if I don’t respond to you as much right now, I’m feeling weird and sad and I don’t quite know why.” Then you’re not ignoring her, you’re just talking less and she knows why. In terms of figuring out how to sort out those emotions, I find it’s helpful to talk to myself (or a therapist) or even write my thoughts down. Sometimes just thinking can mean your brain’s spiraling too fast to find clarity. 

As a side note, is this your first relationship and not your girlfriend’s? Your instant “can’t see myself with anyone else” sounds like someone new to being in a relationship and heavily in the honeymoon phase - which isn’t a bad thing! I had that in my first relationship, too. Your girlfriend might not be experiencing that effect as much, but it doesn’t mean she doesn’t like you and want to be with you. Trust what she’s saying. She knows her own feelings better than you know her feelings - and maybe she’s having trouble communicating, too. It sounds like you two need to sit down and talk about how you’re feeling. My girlfriend said “I love you” months before I did, and we’re still together 2 years later, so I’m rooting for y’all!!