Modesty, and currents in Judaism

I have a few questions, and I would appreciate it if the Rabbi could answer them at length, rather than in a concise manner.
modesty:
What is the definition of modesty? And how should a person conduct himself in his life in terms of modesty, because I'll tell you what – if a person completely abstains from girls (in terms of appearance, not necessarily a relationship. I do think it's not good for a man to have a good relationship with other women outside of his family, if the rabbi thinks otherwise I'd be happy for him to clarify this for me) as is often the case in the Haredi world. So a situation arises where if I (national religious) and he walk down the street and see a woman with a tank top/short shirt/pants/not something too revealing, but for sure yes.
And not only will it be difficult for him not to think about her or look at her, but her appearance will "turn him on" much more than it will me.
It could be because I "eroded," so my question is, is erosion correct and desirable and the ideal, or should we make reservations and boundaries?
I would be happy for the Rabbi to clarify for me, based on what he knows, what he knows about the dispute between Kav and Gush, and what the Rabbi defines himself as and why?