There is the verb 'jemandem verfallen' (to fall for someone). It has a quite ancient and somewhat elevated, maybe even slightly snobbish touch to my ears, though. But I can totally see myself using it in a context you describe:
In Wahrheit bin ich Dir schon vor einer ganzen Weile verfallen.
While deepl or google don't suggest it, it's one of the possible translations listed on leo. Mind though, that "jemandem / einer Sache verfallen" is quite a strong expression and can have a touch of negativity in it as can be used to describe that ones attachment to the person or thing goes beyond a healthy level (like "addicted to someone/something").
It also suggests the much less strong translation of "sich in jemanden verknallen". That is much more colloquial and does not convey deep-running feelings.
Further possibilities are
- ein Auge auf jemanden / etwas werfen
This does not necessarily mean sexually attracted though. Just "you have attracted my attention".
- sich in jemanden vergucken (see answer from @xehpuk)
- Schmetterlinge im Bauch hervorrufen
literally "cause butterflies in the stomach". This figure of speech is used to describe the feeling of elation caused by the presence of the other person.