You’re not wrong, and this sounds just like what happened when I told my parents. I felt pretty bad about it too, so just hang in there.
The first thing you need to do is figure out what you’re confident about. Your profile says you’re 21. Have you been interested in diapers your whole life? If not, how long and in what ways. Do your desires wax and wane? Do you need diapers for arousal (I do, for what it’s worth, nothing wrong with it). This step is important because your mom is worried that having a fetish will make life harder for you. And if you’re not confident about your own needs, she may push on that and try to get you to give up anything that you’re unsure about. So before you talk to her any further, self reflect about what’s most important to you and where you might be willing to give or change. Be open and really think it through.
When that’s done, I’d suggest taking to her again. Do it someplace where you have time and privacy. No interuptions. Some people will disagree with this next bit, but when I discussed it with my parents, I compared it to being gay. This is because, for me, it has been a lifelong thing and a major part of my overall orientation. It also undercut the addiction argument. sex feels great, but nobody with good sense is going to accuse a gay man of addiction to sex with men. You shouldn’t be accused of addiction to diapers. Having said that, if your experience has been very different from mine, you may want to phrase it differently.
The other thing you can do, and be CAREFUL with your phrasing, is to tell her that she’s contradicting herself and tell her what support looks like. The content you want is “you said you support me, but telling me I should stop and seek help isn’t support. This is not something that goes away, and I don’t want it to go away. I told you about it because I love and trust you, not because I wanted help getting rid of my feelings. Support here is accepting that this is part of who I am and figuring out how to live with it going forward.”
Also, a final question. You said you told her because you love her. But why did you tell her now? I told my parents because they were pressuring me to find a girlfriend and I wanted to change the way we approached that topic. That reasoning also made it easy to tell them what support looked like for me: let me be, be okay with who I am, be willing to give advice when I ask. So think about that too. What prompted you to talk now, and how does that feed into what their support would look like?
I wish you and your family good luck.
Edit: oh and be willing to try therapy. You don’t have to, but a therapist telling Mom that it’s okay could easily work in your favor and it might be something you end up liking.