Learn Spanish (Latin America): Durante la terapia, me di cuenta de que repetía el mismo patrón. - During therapy, I realized that I repeated the same pattern.

The verb phrase darse cuenta requires the preposition de to introduce what you are realizing. Even when that realization is a full clause starting with que, you must keep the de, creating the combination de que. Dropping the de in these situations is a common grammatical error called queísmo, though you might hear native speakers do it in casual speech.

Why is repetía in the imperfect tense while di is in the preterite?

You use the preterite me di cuenta because the moment of realization happened at a specific point in time. However, repetía is in the imperfect because repeating the pattern was an ongoing, habitual action in the past. You realized (a completed action) that you were repeating (an ongoing background action) that pattern.

Could I use a form of the verb realizar here instead of me di cuenta?

No, because realizar is a false friend. In Spanish, realizar means to execute, carry out, or make something a reality, like completing a project. When you want to say you mentally realized or noticed something, you must use darse cuenta de.

Why is mismo placed before the noun patrón?