"I only realized" is a perfectly grammatical and usable phrase in written English.
You can use it when you want to express a delayed understanding of something or an epiphany. For example, "I only realized the gravity of the situation after I had already made my decision."
"And I only realized how really good it was when I was preparing for Newsweek".
I only realized it was an event because we had special dresses".
"I only realized that in the hospital, because I was focusing on my kids".
"I only realized what was happening when I was 50, because there I was, making my way in the unconscious rules," Ms. Dorner said.
"Something I only realized in my adult life is that one of the reasons I love writing musicals is that musicals are collaborations.
I'll tell you what his method was—I only realized it later, after working with other directors: he let the actor talk himself into the part.
And so those readings, and our conversations and discussions about those readings, were really formative for me, in ways that I only realize years later, looking back.
I'm supposed to meditate/read/write, then start my day, but if Twitter gets involved, I only realize thirty minutes later, when my phone buzzes yet another reminder at me, that I'm losing time.
I only just realized that I've actually written three stories in succession about innocence: In "The Stain," a young woman is "innocent" about the realities of politics and of life beyond her immediate rural surroundings.
I only just realized what it is the recently-released CX4 reminds me of: one of those disposable cameras.
I only just realized that I no longer judge movies based on how much I enjoy them, but rather by how long they occupy my kids.
