Felicitations on having succumbed to thee madness. Our greatest hope should be that it gets us all in the end. As for the many things that go along with it…that can be complicated indeed.
First: no man is “prepared”. All of the advice given so far is good and worthy of attention, certainly, but none of it, or mine, or that in books will serve to fully “prepare” you for what awaits. Our minds are overcome, and then doubly overcome (and more puzzlingly so) with the arrival of your own children. Fear it though you rightfully may, that terror is normal, and it shall ebb once the child is born and you find a new pattern of life. Though I never wanted the experience, though I dreaded it, I’m now very glad to have it every day.
Aphasia: Step-parenting is about communicating with the lady (What does she want your relationship with her kids to look like? How can she help you make that happen?) and with the kids (What do they miss about having a dad in the house all the time? Can you provide any of it? What if anything do they like about you? What if anything do you like about them?). They have a family unit—they need to welcome you into it, and you need to want to be there. Create boundaries immediately and respect theirs. And always try to speak clearly to each other.
Chaos Theory: Someone above mentioned that you’re not just getting the three of them, you’re getting your lady’s parents in the bargain as well as her siblings, her ex, and her other family, and that is very true. Your own existing family are also going to be mixed in. Your family will be happy and excited for you and then elated when your own children arrive. It will feel chaotic and strange to navigate at first (and maybe for a while). Whose parents do you spend the holidays with? How often do you want to see family X even though they live nearby? Are certain family members just allowed to come by any time? How do vacations work? Agree on rules about everything with your lady or the chaos will overwhelm all of you.
Time travel: Being around children as a parent (or even a step-parent) does this neat thing that no one told me about. You have moments, often fleeting, where you flash to a memory of your younger self that you may have all but forgotten. You also notice small things in your kids that remind you of your partner, or of some family member. It is a little like getting a glimpse at something about yourself that was otherwise lost to time.
Transmutation: I hate to tell you, but once you are a dad, your previous self-identity crumples. Yes, the identity that you’ve spent all these years creating. It can be unpleasant to feel it, and it can cause a lot of doubt about whether you, the real you, is gone for good. But it comes back. Slowly, facets of your old true self start to re-inflate and fit together with the new parent-self in ways that are surprising. And things you maybe didn’t like about yourself can be left deflated and cast off more easily somehow. You can sort of become a better version of your self than you had ever imagined. I don’t understand it, I just know the experience of it.
In the end, you are lost to madness. You cannot change it. You know this. Accept it and do all you can to define your new reality, and know it will be shattered again (and possibly again, depending upon your breeding aspirations).
We’ll all be here to monitor your dwindling sanity, worry not.