I don't negotiate well - less because I lack confidence, but more because I tend to put everything on the table and expect people around me are doing the same. I don't like not being honest and open, and don't like interpersonal interactions that are constructed as having a "winner" and "loser". So I tend not to negotiate about things - ignore things that don't fit my needs until I can do better somewhere else.
But I know it's something that affects me negatively sometimes, and that one contributing factor to why women in general are often worse off professionally than men with similar skills and backgrounds is because
they don't negotiate, for a whole variety of reasons. I just got a job (glory, hallelujah), and during pre-offer discussions we talked about what my "absolute minimum" salary would be. I gave them a number in the mid-range of another job offer I had on the table, and when the new offer came through, they gave it at the "absolute minimum" number. Although I felt bold and pushy, I countered with a number $5K higher, expecting to get negotiated back down a couple of $K. But they DIDN'T. Because I went ahead and ASKED, my starting salary is now $5K higher than it would have been. (And because this is in the higher ed context, that's more significant than it would have been in law.)
Lucky enough to have gotten the job, but really glad I asked.