Gordie-- sounds like you're making great strides in understanding why your marriage is falling apart. I think most often it's not just one thing; it's a mix of things:
(a) the things we husbands did that just angered/frustrated the wives for years; (b) a midlife crisis; and (c) the wives' personalities/personal histories.
It's good for us to work on (a) but (b) and (c) ... will take a long time to play out and resolve, if at all. Maybe this is an obvious point but I guess I just want to say it's good to try to see what is an issue that is a life-long issue (i.e., (c)), vs. something confined to just your marriage (i.e., (a)).
Just thinking aloud, as reading your situation helps me think about mine.
As for the V-day gift ... my vote is to give her something thoughtful that reflects your warm feelings for her, but nothing explicitly sexual.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final