I boxed a lot when I was younger, was part of an ABC and had a fair few fights at amateur level. Was mainly driven by my Dad who was a professional boxer back in the day, but aside from the fact it got me through my teen years without being victim to bullying I never really enjoyed or liked it.
I joined a new club a couple of years back purely for fitness reasons. You can't really expect to achieve anything by just hitting the bag, the sport requires a lot more from your body than just your throws. You need to be skipping for the agility, doing intensive circuit work, running, sparring and training with a coach on combinations and defence.
I think probably the most over-looked factor of boxing though is the fact you get punched in the face. Speaking from experience, you get punched in the face. That pretty much defines you as a boxer or not. If you get knocked out it's fine, you're on the floor and you've got nothing to worry about, you're out cold so you don't feel a thing. It's being punched ****ing hard in the head sometimes repeatedly and shaking it off, keeping your composure, controlling the red mist, keeping your focus and jumping back in. I had maybe 5 or 6 amateur level fights (lots of sparring but it's not the same), they honest to god require a **** load more mentally than they do physically. People have this idea (and I myself do the same thing), they picture themselves in a fight with someone and they visualise themselves doing some kind of jet lee **** to someone else, not getting touched and flooring the other guy (perhaps looking awesome cool in the process). But the dirty truth about fighting is that you actually get beat up, you don't really ever win a fight unless you're A) fighting someone who doesn't have a clue about fighting (pretty much women, for men it's instinctive although it may seem they're windmilling) or B) you've got a knife or a gun.
But yeah, back on topic, if you want to get into it find a club. You mention stress/anger problems, you might want to have a fight with someone first and see if you don't red out. A lot of fights are dictated by one party getting so angry they go into what's called the red mist, which is basically a fit of rage where you lose rational control of yourself (you might have experienced it to a lesser degree if you've ever been tied up and wanted desperately to break free as you felt trapped). If it happens to you you've pretty much lost.