I will briefly share my 'Professional' gaming experience since 1990 (i.e. when I started getting paid to do it, not since I began doing it).
Overall, the places where I had a lot of creative input into the product were happy times. If they said 'We wanna...' and left me to it, they 'got a', and I was always happiest.
The places like EA/Tiburon where it was a soulless corporate crappy-game-factory that expected extra hours every day and pieces of every weekend to 'make good' bad planning on management's end, well, I'm glad I was laid off or I'd have had to suck on a shotgun barrel. Absolute, blackest misery.
As a contractor, it's a really mixed bag. I find I usually have at least bosses and bosses' bosses for two or more levels of subcontracting, with me at the bottom doing the actual work. At one previous place (Digital Eclipse and its various spawn) I discovered third hand that after I successfully shipped half a dozen games for them, that I was accused of being 'volatile', and so they stopped using me. You know, it's a bit hard not to be 'volatile' when your milestone invoices are consistently three months behind being paid, they keep adding new things to the game that delay milestone completion, and no matter how many examples and tutorials and explanations I give, I still get 24 bit RGB art that uses all 24 bits to fit into an 8 bit color indexed Gameboy Advance, and whiny 'feedback' from them about how I'm stripping colors out of their multi-thousand color art (times all the pieces of art with different sets of multi-thousands of colors) in order to make it work. Not that I expect artists to add, but they're supposed to have worked on these kinds of games before and know better than to give me PC art for a hand-held.
I've found I'm the happiest doing my own thing. Sure these cheesy Flash games I'm getting paid to do aren't much, but they're MY 'not much'. At least until near the end where the various stuffed shirts at the publisher's end get around to trying to put their own personal thumb-print on the game, so they can feel as if they contributed something besides noise.
As long as you can tolerate working on a game with a credits list that takes 15 minutes to scroll through, while knowing it was just you programming, one or two artists and maybe a sound guy and a producer/manager who got the game done, you will have half of the right mindset for the industry in general.
Thinking back, there is no single game I could actually point at that I was paid to make and say, "I'm really proud of THAT!". I've shipped lots and lots of games, and few of them have been the sort I'd really spend any time playing, and of those I was so wretchedly sick of playing them by the time I shipped them that I couldn't even bear to look at them. It's just what they wanted, but not what I wanted. So as long as you can stomach shipping crap you hate for money, you have the other half of the mindset to be a TRUE professional.
My biggest lesson is, being an American, get your own health insurance and pay for it yourself. In the gaming industry you will have no job security or stability, and as you get older it gets ever harder and more expensive to get health insurance as a contractor or independent developer. When you're 20-something, health insurance costs about as much as car insurance on a new Honda and is easy to get. When you're 40-something, health insurance costs as much as your mortgage payment, and you're stuck in COBRA from your previous job and eventually using HIPAA to keep the insurance going, because when I was younger I were stupid enough to get sick once and see a doctor about it. Lock in a rate on good coverage when you're younger, and don't take the 'free' (typically pretty lame) insurance your various employers offer. It won't keep the insurance from getting more expensive over time, but it will keep it from running out of control.
