So what you have here is, ultimately, a failure to communicate.
You say you told her when you started your relationship and she let you know that she didn't want to be a part of it. That part is fine: good, clear communication. But what has happened since is not. You have allowed a basic human need (your own desire for comfort through a fetish that, though she doesn't understand, is in fact an essential element of you) to retreat and hide and fester. In doing so, you've created a situation that is unhealthy for your relationship: you are "getting cranky and upset" and beginning to blame your girlfriend for it. Though you care for her and "don't want to disappoint or offend" her, the truth is that you've placed yourself in an untenable position: things will only continue to get worse until you talk it out.
I know that's probably not the advice you want to hear, as it is likely to be the hardest thing for you to do. It may jeopardize your relationship. She has already rejected your fetish once. On the other hand, your current level of anxiety is evidence enough that continuing the status quo is simply not an option. So tell her your problem. But prepare. Do it in a thoughtful, adult way. You are her boyfriend, after all, not a baby seeking comfort. Make her dinner, or take her out; this has to be handled in an adult setting. Tell her how much you love her (if that's where your relationship is), and then explain that you don't want anything to come between you. When you bring up the diaper stuff, be sure to acknowledge that you know she has told you she doesn't want to be a part of it, and you respect that. But explain to her how much it is a part of you. Tell her it is a harmless thing that gives you pleasure even if, at first, it seems totally weird. Tell her about your current level of anxiety caused by the fact that you are leaving it out of your life for her because you are always together. If she responds by asking whether you'd rather not be together, emphatically deny this, but tell her you need some opportunity in your life for the diapers or you'll implode; surely she's noticed you've not been yourself lately, right? She might ask if she's not enough; tell her of course she is enough...for your love, for your relationship, for all of your world. But there is a single deep need inside of you that needs to be satisfied once in a while. Maybe you'd prefer her to be a part of that; it would be perfect that way. But though she doesn't want to, you still need it. And ask her how to help you work out opportunities to do so.
Maybe she'll suggest that she can go out with friends. Maybe she'll offer to try it with you once. Maybe she'll be disgusted and leave you. It's impossible to know. But you really don't have an option here: you have to talk with her about this.
