I will speak from experience.
With the first two women I had sex with, I blew my load within thirty seconds of penetration. During that period, I wore briefs all the time.
Then I decided to freeball it for a few years. Actually, I've been freeballing it ever since.
From then on, I last as long as I want to. Coincidence? Don't know. All I remember is for a few weeks when I first started freeballing, I tended to get unwanted erections quite frequently just from the head of my dick being able to swing freely and rub against the inside of my pants. Eventually, that went away, and I can say that I feel generally less sensitive now than I did before.
The benefit to being less sensitive? For me, it means I'm able to control my urge to ejaculate, while before there was nothing I could do to stop it.
The only other method I can think of to desensitize your dick would be wearing multiple condoms. Hell, slip three or four of them on, and even keep adding them from there until you can't feel a thing. If your girlfriend gets suspicious and questions what the fuck you're doing right about the time you're unrolling the 20th rubber over your shaft, just come off with something like,
"Oh, BELIEVE ME, baby, by the time I'm done pounding you, at LEAST fifteen of these will be torn to shreds,"
And if she doesn't run away screaming "fucking weirdo" at that point, then at least you know she's got a sense of humor and you'll have tons of fun finding out exactly HOW kinky she's willing to get.