Fatherhood won't change you.
I'm going to make an unpopular statement. Fatherhood will not change you. Parenthood is not a salve that, when applied, adjusts aspects of your life into more orderly forms. People will often say, "When they become a father/mother, they'll really grow up." and this is, occasionally, true. What people fail to recognize is that it isn't the simple fact of becoming a parent that elicits this change, it's the response an individual has to the massive adjustments parenting requires. A juggler who is competent juggling three balls at once will not miraculously become a better juggler if someone tosses them a fourth ball. They will either, with time, learn to juggle more or drop them all.
The idea that parenting will inherently change someone is deeply flawed. Someone has to want to change well before they're ever going to on their own and it is dangerous to lead people to believe that when an infant enters their life they will be imbued with ancient wisdoms and previously unknown secrets as a result of their entry into parenthood.
Like every other major life event, entering fatherhood gives you a choice. The choice to look at your new circumstances and continue living life the way you had or to attempt to adapt and become something more than you were before.
For me, that meant an abundance of change, but that didn't come easy and it certainly didn't come because I was suddenly a parent. Luck favors the prepared and I have the good fortune to have a propensity for planning. I spent the early parts of my wife's pregnancy consuming every relevant book I could find on the topic of becoming a father; once I discovered we were having a boy that shifted to a mixture of fatherhood, raising boys, and the changing definitions of masculinity in our society. I didn't just want to be prepared, I needed to be if I was going to feel remotely comfortable for what was in store for my expanding family.
The reason I bring all of this up is because I had plenty of people who told me about how "it all changes when you're a parent" and how "when you hold your baby for the first time, you'll just know what to do." The first statement was true, but not in the way they meant it and, when I held my baby for the first time, I was terrified at the sudden realization that my hundreds of hours of study had done little to equip me for the journey I was on.
If you want to be a good father, and I don't claim to be an expert myself, it takes work. People say parenting is a full-time job and no one is equipped for their career through the magic of donning a suit and tie. They become good at what they do by putting in the time and forcing themselves to adapt and evolve as people. Fatherhood requires that of you and so much more, but the change must always come from you.