Learn English – “I didn’t realize”

Both tenses are OK, but I believe the past tense is a bit more common: it may be somewhat contrary to logic, but it sounds better. Harmony of tenses (if that's what it's called) is a linguistic phenomenon that is not always very logical.

I think the important thing to keep in mind for a non-native speaker of English is that when the reference time is in the past, there are two ways to render a habitual meaning: with a simple preterite form, and with the compound past continuous form. So in general, both of the forms should be correct.

This is true, however, only for verbs whose meanings involve an action which has a palpable duration like work, live. e.g.,

1. When I lived in Utrecht, I used to love to take walks.
2. When I was living in Utrecht, I used to love to take walks.

For verbs with punctual meaning, like fall, only the simple preterite form is appropriate for a habitual meaning, and whenever is more appropriate than when. e.g.,

1. Whenever I fell, Thomas would help me up. (refers to a habitual event)
2. ?When I was falling, Thomas helped me up. (sounds odd, because it would refer to a single act of falling)