It will be different for you based on lots of different variables, and just by virtue of the fact that you are on MDC, I think you have a leg up on where I was when I had my first, but here is my story:
Going from no kids to one was the most difficult and painful experience of my life. I was 22, barely married when we got pregnant, just moved away from home, and had no friends or family to speak of in our area. I had absolutely no knowledge of childcare, parenting, or birth/breastfeeding, and I felt like the information that I had from my mom and MIL and my care providers was sufficient. I was afraid to ask questions and to question authority.
And then DS came along. Apart from having a labor and delivery that was exactly the opposite of what I wanted, DS was exactly the opposite of what I thought a baby should be. He screamed constantly. He didn't gain weight. He nursed all day every day. He woke every 45 minutes for the first year. He met all his milestones late (albeit in the range of normal). I felt like everything I was doing was wrong. I felt like I had failed labor and delivery, and failed breastfeeding because I had to supplement, and failed at being a mother because he was so unhappy all the time. I was barely functioning. I did not know how to get help, and I was afraid to get help because of mental health stigma in my family. My husband essentially dragged me through the first year. I parented based purely on animal instinct, which led me to attachment parenting. (We have since learned that he has special needs, which explained away a lot of the pain we experienced in his first year, and has helped relieve a lot of my guilt.)
I never thought one little person would turn me so completely inside out that I would become an entirely different person. My former life seems like a memory, but not my memory.
I think parenting makes you, overall, less selfish, but for me, it made me more selfish, and it was a welcome change. Before DS, I never knew what I wanted or needed, and if I did, I was too afraid to ask for it. I have learned how to advocate for my own health and well-being and to recognize my own needs, just as I have learned to do so for my son. I have learned that my needs are valid and equal to those of my spouse and child, and that there is no harm in asking for something. I have learned to be gentle and forgiving of my own shortcomings and the shortcomings of others. I have learned that I absolutely cannot do it alone.
Having a child also forced me to look at my own childhood through a different lens, and what I saw made me very sad and surprised. I thought I had had a good childhood, but when I wanted my mom to be understanding and loving after DS's birth, she was cold and judgmental and manipulative, and I realized that she had always been that way. My relationship with every person in my life went through drastic changes, and my relationship with my parents is the only one that hasn't recovered and grown to maturity since I became a mother.
Your perspective on almost everything will change, and in some ways you will feel zealously focused on one thing, and in some ways you will learn how to balance.
Things I miss:
-I am entirely a creative person, and I miss spontaneity that seems to fuel that. My life can be chaotic and mundane at the same time, but it is a real creative sap for me. It took a year to write a single word after DS was born, it's taken 10 mo with DD.
-I miss being able to leave my "job" behind when I take time to myself. I went to my first post-DD movie the other day and I could not stop thinking about how LOUD it was and how someone was sure to wake up

.
-I miss having sex in my own bed. Yeah, co-sleeping forces you to be creative, but sometimes I just want to be boring about it.
-I miss being able to run quick errands, or multiple errands at once.
-I miss being able to sit through church, a meal, or a conversation without constant interruption, and I hate that some of the interruption is coming from my own scatterbrained thoughts. I wish I could complete a sentence!
-I miss being able to focus on one thing at a time.
-Being able to eat a slice of cake without someone begging for over half of it.
-Being able to eat slowly and thoughtfully, instead of cramming everything in, hoping to finish before someone at the table goes completely bonkers for the 1143rd time that day.
Things I love:
-The first time my son said "I love you".
-That chubby, hot hand that just somehow finds yours when you're out for a walk.
-Gummy baby grins.
-I have become the world's BEST multi-tasker!
-Seeing the world as a completely new and incredible place, and falling in love with God's creation all over again.
-Knowing that, despite the struggle and the resentment I am still trying to let go of, my son has made me into a much better person.
-Seeing the joy that my child brings to my spouse, my parents, my in-laws and even complete strangers.
-Singing all day to someone who loves to hear me.
-Snuggling up to a warm baby at night.
-Breastfeeding.
-Watching a brand new and completely original personality unfold.
-Letting go of life's mishaps and messes and living truly in the moment.
Just remember that it will be a completely personal and original journey for you and your family, and remember to reach out to others when you need help.