I feel that the answer here is to keep the lines of communication open and to be reasonably willing to find compromises together in effort to maintain a healthy relationship between adults. She's told you her feelings on the matter, and you've told her your feelings, so it's a good time to work together on finding the meeting point in the middle where you both have happiness. I don't feel this a black or white situation, and that there are plenty of happy, fulfilling options to consider together that don't mean you are "settling" in an unpleasant way.
Working together with your partner to find a mutually beneficial arrangement when it comes to sex is not necessarily "settling" or something you should avoid. You don't have to blindly overstep your own boundaries just because your partner's boundaries are different, but perhaps you can explore exactly where your firmer boundaries are right now and keep in mind that revisiting those lines periodically can be helpful in long-term happiness. You can work together through this, and you can still find happiness if you willingly make those efforts. I doubt your partner is asking you to forego your current regression space and be an entirely sexual little, and I doubt you want to end the relationship just because one thing between you two doesn't align exactly as you had hoped. Your partner is not an evil monster seeking to hurt you so you don't have to think of this as a you vs her situation.
My suggestion is to talk with your partner about this and to try to come to an agreement that works suitably for you both if you want to continue the relationship. If you want to make this work out then you can do it as long as you are willing to put in the time and effort to work with your partner in meeting one another's needs. Relationships are give and take and sometimes that certainly means you must be willing to work with your partner to meet both of your needs fairly.
Start by calmly asking your partner:
- Why it's important to her that you are regressed during more mature activities together so that you can gain her perspective and understand her feelings.
- How deeply regressed she would prefer and what exactly that looks or sounds like to her.
- What is the minimal amount of regression that could suffice in making the situation accommodating to her as well.
- If there is an alternative method to active regression during this activity, and, if so, what you could potentially do to meet that need instead.
- If there are any sexually fulfilling situations or activities where regression is not an important factor for her.
- If she feels that regression will likely always be a mandatory part of her sexual fulfillment or if there is the possibility of that changing over time.
It's okay to have boundaries and limitations, but you should also listen to and consider your partner's perspectives. I'm sure that if you begin talking more about this subject that you will reach some agreements that will make the situation work out well for you both. Try not to rush too fast into making any agreement just so that you can have a good experience (there's no need to rush so really do take your time in considering options you think of together
), and try to keep in mind that such an intimate experience should also be positive, comfortable, and enjoyable for her as well. Relationships take effort if you want them to be successful and fulfilling, and it's important to keep in mind that it isn't only about you and your feelings any more.