We get questions like this every now and then. I have a lot of personal experience in this regard, I've told numerous friends who responded positively and I have had 2 girlfriends in the past, both of whom I told, one of them thought it was fine, but she let me indulge on my own time, she wanted no part of the Mommy/Baby dynamic and my 2nd girlfriend was completely OK with it and steadily began mothering me when in baby mode.
First off, it's totally natural to be a little nervous, I think many of us AB's are pretty self-aware and we know that being a big baby and or enjoying diapers can be a bit of a tough sell for a prospective partner, as such we tend to get iffy and protective when it comes to revealing this side of ourselves to others. Eventually, as you get closer to your partner and more comfortable with this side of yourself, being nervous about telling will fade fast.
Before I proceed, can I ask you a question as well, how long exactly have you been dating this person? I ask this because when you decide to tell them usually matters. I've heard of people who confided in their partner extremely early, a few dates in and it appears that if you tell too early, you run the risk of coming on too strong and driving them away. I've also heard that if you tell them too late, say years into the relationship, they respond poorly because it's almost like you were deliberately hiding a part of yourself from them, it causes trust issues.
I told my first girlfriend 6 months into our relationship and my 2nd girlfriend 3 months into the relationship, things were serious and we practically did everything together at those points in the relationships. So, from personal experience if your relationship is between the 3-6 month mark, that's a pretty opportune time to tell, it's still early enough to break away without any hurt feelings if it doesn't go over well, but not late in the game enough to risk severe disappointment or a possible break-up.
Secondly, the type of person your partner is should help you determine when you tell as well. I waited with my first girlfriend because she wasn't a big child or baby person, she was very independent and stern, more of a rock in the relationship, not the life of the party or the best friend, I figured if I told her too early she'd break it off right away. I waited a shorter amount of time with my 2nd girlfriend because we had a lot in common, she had a very bubbly demeanour and had a strong maternal side she was very upfront with me about, she worked at a daycare and an animal shelter during her time in University, so I figured chances are she'll likely be OK with this side of myself and she was, more so than my 1st girlfriend.
When I told them, I tried not to make things awkward. I prepared what I wanted to say beforehand and I treated the day like any other, I invited them over for a dinner, we got to talking and hanging out and then when things quieted down and we settled in for the evening, I slowly started the conversation. I told them that I was an AB, what being an AB means to me and how important this was to me as eloquently as I could without giving away all of the details and then I let them engage in the conversation as well, I gave them time and space to ask their own questions. If you prepare how you're going to bring it up and what you're going to say beforehand, you can effectively reveal this side of yourself in a way that is devoid of nerves and awkwardness. If you give them the time to ask their own questions, the conversation feels like a mutual one, it becomes a conversation that builds trust and makes things less one-sided.
If your relationship is still fresh and you don't know where you stand or what being an AB/DL means to you exactly, then maybe it's best to take some time and figure things out further. When you are comfortable and confident, prepare what you want to say, have a date like any other and bring it up when things feel right. Hopefully, your partner is an open-minded person and they will accept you for who you are. Chances are if you have a lot in common and similar interests they'll be fine with this.
Also, if things go over well don't expect them to want to take part in this right away. Even after you tell, it'll take time for your partner to fully come to terms with this as well. My 1st girlfriend never came around to embracing my AB side fully, but didn't judge me for it, she allowed me to wear and regress in my spare time and my 2nd girlfriend was OK with it from the get go, found it cute, but it even took her time to get comfortable with caring for me when I regressed.
Lastly, there's no need to rush things. Asking within the 3-6 month mark, preparing what I was going to say and starting the conversation in a very natural, straight forward way worked for me, but that doesn't mean that'll work for you. Only you know your partner and the nature of the relationship well enough to know how they will react and when the time is right and only you know what being an AB/DL means to you, so you'll have to figure out what it is you're going to say when the time comes. Hopefully, my experience can be of some help to you and I'm sure others will chime in with their support and advice as well.
Whenever you decide to tell, I wish you luck
PS. I just got my PeekABU's Thursday, but haven't been able to wear them, I'm preparing for a trip across the country. Oh well, they'll be waiting for me when I get back