Hello, I'd like to help you with your question.
It is understandable that your girlfriend is concerned with the overwhelming task of caring for your child the rest of her life. It is stressful for you no doubt, but your girlfriend is not only starting a new relationship, but she is taking on the responsibility of a dependent adult. So her concerns are valid. She may want to be sure she is emotionally and physically able to be there for your child.
To help her, first understand her point of view. Talk to her at length about her feelings and allow her to come to you with any fear or concern she has. She is going from no children to a dependent adult and that would be overwhelming for anyone. So letting her vent any feelings she has without judgment would provide her a way to express her concerns.
Make your relationship with her primary. You do have to care for your child, of course, but make sure your girlfriend knows that you are putting her needs first, as much as possible. That way, she does not feel your whole focus is on your child and she gets left out.
Be sure to build in as much help as you can into your daily lives. Your girlfriend needs to know that the total focus for both of you won't be the work involved with caring for your child and that there will be help and breaks.
Focus on your relationship by planning trips alone, nights out and frequent special times together. That will help her feel special and give her time with you alone.
If you feel at any time that the stress
of care is getting to your girlfriend or you, consider counseling. Caring for a developmentally challenged child can be especially stressful. Counseling is an excellent outlet in a safe environment. And it can provide additional support and answers to your situation.
I hope this has helped you,