First off, congrats, I'm glad you were able to find someone so accepting and willing to be your Mommy
Secondly, I think the best advice I could give in this instance would be to build rapport and trust with this person and to take things slow.
Having had a GF/Mommy in the past and a Mommy friend, starting out, it was always a little nerve-racking to regress in front of them, it wasn't until trust was built that I felt confident being a baby around them and my nerves abruptly ceased. It sounds as though a nice foundation of trust and rapport already exists between you two, as you've been getting together for some time now, so on this front, you should be good.
Taking things slow also helps because as new as this is for you, it is new for your Mommy as well. Introducing new things into an established dynamic is also going to take some getting use to for the both of you, the same with just approaching a Mommy and Baby dynamic for the first time. When my second GF wanted to Mommy me, we started off extremely slow, our first few Baby/Mommy times together consisted of nothing more than her putting me into a diaper and cuddling me, we built up from there. With my Mommy friend it was largely the same, our first few dates together revolved around diapering and cuddling. The diaper changes, feeding time, bath time, nap time, story time all of those things came later. When you take things slow, I find not only do both of you learn to trust one another more, but you come to understand one another more as well. As a baby you'll become more in tune to what your Mommy wants and vice versa. It was taking it slow with my Mommy friend that made it obvious to me that she was getting just as much out of our dynamic as I was and this realization allowed me to release my inhibitions more and enabled me to be completely vulnerable around her. So your little boy will become more believable as long as you two build rapport and take things slow.
Another thing that can curb nervousness is simply practicing this dynamic with greater regularity. The more you regress around her and the more you share time with one another as Mommy and Baby, the more comfortable the two of you will become, hence the nerves will eventually fade.
I hope this helps. It's always hard giving advice on these matters as my Mommy Friend and I recently parted and every dynamic a fellow AB has with their caregiver is always going to be personalized and thus very different from my own. I still think these basics apply; build trust/rapport, take things slow, practice makes perfect. Stick to those essentials and all will be good, the nerves will cease and your time together will become increasingly believable/real.
Best of luck