Kara: I mean, CIark seems Iike a reaIIy nice guy
and everything...but I can't heIp but feeIing
he's hiding something from me.
She keeps feeling...?

Lex: Heh. As his former best friend,
I know the feeIing aII too weII. Very well?
Kara: See, (Does 'see' mean here you see what I mean?
or is it just 'and' as in And he didn't mention that?)
CIark didn't even mention
that you guys were friends.
IMO "I can't help but feeling" is incorrect grammatically. It should be either "I can't help but feel" or "I can't help feeling." Either means "despite myself."

Although I love to eat veal, I can't help but feel sad when I think about the way calves are killed.

"All too well" means, literally, too well - you wish you didn't know whatever it is quite that well.

I understand how to live on a budget all too well. My husband was out of work for eight months last year, and we were almost evicted from our apartment.

"See" - here, I think it means "look at this example." Or in this particular case, "well look, what you just now said is an example of what I mean!"
Thanks, Delmobile!