You can't avoid situations where confidence matters. In most type of negotiations confidence matters.
And how would a lack of confidence due to not doing martial arts manifest itself?
As far as I can tell, about the only things in life (after high school, at least) I ever wanted that I didn't get but would have if I had been more confident probably were hookups with drunken bimbos in night clubs (which now that I'm in a relationship is a moot point), and even there being more confident would also have increased the number of times the bouncers threw my ass out of the club.
Looking at how people more confident (IIUC what you mean) than me (controlling for age, intelligence, socio-economic status of parents, and the like) fare in life, I'm under the impression that more confidence would just make me come across as arrogant and make everybody important dislike me. I know people who got into lots of trouble because of that.
Carrying pepper spray into a salary negotiation won't make you more confident to ask for a raise.
And knowing martial arts doesn't change the fact that if you demand too much they'll hire someone cheaper than you. (And in many kinds of jobs the wage is fixed beforehand and they decide whom to hire based on a competitive examination.)
Many people can point to significant events that improved their lives in a positive way. They often refer to these as "lucky breaks", and take it for granted that such events are rare. But most of the time "lucky breaks" don't need to be uncommon-you can often reverse engineer the reasons behind them and cause them to happen more frequently. So when a one-off event ends up contributing a lot of value, you should systematically make it part of your life.
Example 1: in June the Less Wrong - Cambridge community held a mega-meetup with several people arriving from out of state. Since several of us had to stay up until 2AM+ in order to meet with people, we decided to have a game night that evening, which I held at my place. The game night was excellent-plenty of people showed up, we all had a lot of fun, and it was a great way to socialize with several people. Since it went so well, I started hosting game nights regularly, eventually converging on one game night every two weeks. This was a phenomenal move in many ways-it let me meet a lot of interesting people, deepen my connections with my friends, quickly integrate with the Less Wrong community, and just in general have a lot of fun, simply by taking one thing that worked well and making it systematic.
Example 2: a while back I was given an assignment to set up a scalable analytic architecture to allow data scientists to iterate faster-a project where I had no idea what to do or how to start. In desperation, I reached out to several people on LinkedIn who had experience with similar projects. Some of them responded, and the advice I got was incredibly valuable, easily shaving months off of my learning curve. But there is no reason for me to only do this when I am completely desperate. Thus I’ve continued to reach out to experts when I have new projects, and this has allowed me to avoid mistakes and solve new problems much more quickly. This has significantly improved my learning speed and made a qualitative difference in how I work. I no longer dismiss potential ideas simply because I have no idea how to implement them-instead, I now talk to experts and figure out roughly how difficult those ideas are, which has allowed me to solve several problems I would have dismissed as unfeasibly difficult before.
Example 3: a few years back some of my friends in the tech industry mentioned that Machine Learning was becoming a trend, so I took two weeks to learn the basics. A few months later the "Big Data" boom exploded, and I was able to get a job as a Data Scientist at a significantly higher salary doing more interesting work. Even though my Machine Learning knowledge was pretty rudimentary, I was able to get the job because demand completely exceeded supply at that point. In short, this was a lucky break that greatly advanced my career. To systematize this I simply continued to keep an eye out on big trends in technology. I've read Hacker News (which is generally half a year or more ahead of the mainstream), kept in touch with my friends on the applied side of academia (which feeds useful techniques into the industry), and just generally kept talking to a lot of people in order to keep up-to-date. This has been useful again and again, allowing me to focus my learning on the most valuable skills right as there was market demand.
In short, one of the fastest ways to improve your life is to look at things that already made a big difference before, and cause more of them to happen.