You can't really make someone be who they aren't, and you shouldn't pressure your partner into parenting you if their heart isn't in it, but it sounds like the issue isn't so much that your partner doesn't feel like a Caregiver but just doesn't feel they are capable of fulfilling your needs. Perhaps your partner is going through a bit of
Caregiver burnout and needs some quiet time from caregiving. A sudden, drastic change like this is a huge sign that they are feeling the burnout so I would encourage you to read over information about
Caregiver burnout and to talk with your partner about it, bringing up the possibility that they may be suffering from it currently so that you can talk about ways to help them. A Caregiver who experiences burnout is not bad, wrong, or abnormal, and it doesn't mean they don't love or care about you any more.
Note: Even if your partner isn't sure or doesn't believe they are feeling burned out you may want to proceed with making the assumption it is most likely because they may not even realize that this is what is happening to them. Sometimes we just need time to rest from giving so much of ourselves, and there is nothing wrong with that need. Sometimes we don't realize how tired we've gotten or how much rest we need, and that obviously applies to more than just caring for others. Sometimes we just need a supportive partner who stands by us no matter what, and gives us time to heal and come back to being our ourselves again without us feeling pressured to perform like circus monkeys.
Try not to focus on the idea that you have lost something. If your partner has been able to be a Caregiver to you previously then it's likely that it's truly
who they are as a person, just as you are a little by personality and not choice, and that part of them isn't likely to actually just vanish away for good. Since there hasn't been a break-up then that tells me that they do love you and do care about you. Try to redirect your focus to see the smaller things they do that is essentially taking care of you and showing you love. Sometimes seeing the smaller things is what can get you by if they just need a break. Focusing on the negative is just causing you unnecessary distress.
Remind yourself that you do not
need a Caregiver to experience more deeply regressive moments. You are who you are! You do not
need your partner to participate in regression for it to be a positive experience. You don't need to focus on the idea that you are "alone", and you can use your imagination to make believe that your Caregiver is just sitting in the next room, washing laundry for you, or taking care of other household chores while you're playing and enjoying just being little. Your imagination is a tool you absolutely should use. While it may be nice to have another person's support, encouragement, acceptance, and attention it isn't necessary to be able to have fulfilling experiences. You can still work towards enjoy your more regressive times, and you can absolutely adjust to having more private regression while you and your partner works through their feelings of Caregiver inadequacy. I would suggest basic methods of self-encouragement such as rewarding yourself for not becoming overly upset and setting aside personal, private time to allow yourself to work through your feelings and find fulfilling regression experiences again.
I am sure you can work through this and find happiness and wholeness again. Give yourself both time and patience to settle into these recent changes, and gently work with your partner to help them through their struggles as well. Sometimes a little just has to do more big boy or big girl things for the people that they love most and that does logically involve self-sacrifice, patience, accepting change, and giving back to those who gave to them. You can do this.