I would like to help you with your question.
I am sorry that you feel conflicted about your role as mother and your desire to be free to experience the world without the responsibilities and demands of parenting. Parenting is one of the most demanding jobs any of us will ever experience. Children are a delight...but yet the energy and attention often feels overwhelming.
From what you have written, you are being pulled in several directions. There are the older two children who are at an entirely different developmental stage as your 3 1/2 year old. Trying to balance the demands of each can not be easy!
Then there was the relief or freedom you felt believing that your days of parenting were coming to a close end...and there you are pregnant and having to gear up again to care for an infant and begin another new cycle of parenting.
The feelings you describe - irritability, disengagement, depression and the like - are about the conflict you feel and the shoulds and coulds that fill your head. Yes...ideally you would be thrilled and joyous being the mother to your 3 1/2 year old. Yes...ideally you would be done with parenting and pursuing all those other interests that were put on hold.
Rather then seeing yourself as selfish and beating yourself up...it is important to accept the truth, that is, that indeed you are not living the ideal life you so wanted. This is your truth. It is your truth right now at this very moment in your life.
This truth is neither good or bad. Rather..it is.
Is it okay to want an ideal life? Most certainly it is. Is it okay to feel selfish and want to run away. Most certainly it is. Is if normal to want to be free to pursue other interests besides motherhood. Yes, it most certainly is.
It is quite normal and reasonable to want something different than what we have. Perhaps you can think about it this way...if wanted a different hairstyle...would it be okay to be disgusted with the haircut you now have? If you wanted to wear a dress but there were no clean ones in your closet, would it be okay to be disappointed...even angry about that? I do not mean to minimize parenthood, but truly life has it disappointments and we must work to accept those disappointments and adjust to what is present in our life.
It does seem that you have worked very hard to make accommodations so that you can manage your disappointment. I do sense depression here and wonder if you are still taking medication and, if so, if you feel it is having any impact on your mood. I also wonder if it might be helpful to get back in therapy and continue working with a therapist.
I would be happy to chat with you.