could someone help me..please

i am confuse when using "have" and "had"

On my own understanding :

I haven't done that. = i never done that before up to present
I hadn't done that. = i done it before but not up to present

Imagine that i am talking to my friend right now
should i say:

before my girlfriend came into my life, i have never climbed mountain.
or
before my girlfriend came into my life, i had never climbed mountain
= the situation is i didn't climbed any mountain up to now/present

ANOTHER is, should i say:

If you were not here. I could have done that
or
if you are no here. i could have done that
(imagine that i am talking to my friend right now and my he is right beside me.)

.