My twin girls are now thirteen years old and one thing I have learned, is that routines change right when we get comfortable. As the children move through their different development stages, their needs change, and we, as parents, have to adjust.
Nobody knows better than you what your needs, expectations, and limitations are. If you find yourself questioning how much more you can handle while maintaining a sense of well being and balance, then you may want to give yourself the time and space to think and plan carefully.
You seem to know yourself well. Trust your instincts, share your thoughts with your husband, and evaluate your support system. Remember, when mama is happy then everyone is happy.