It took me twenty years to decide to go to law school after working as a paralegal for that long. Here's the deal:
1. You will work very, very hard. Your life will be dictated by billable hour requirements and deadlines on your calendar. You will have to sacrifice a lot.
2. Your clients will at times turn on you because you are often the messenger of bad tidings.
3. You will hold the lives of your clients in your hands (financial lives most of the time). The responsibility will keep you awake at night.
My opinion on the key to happiness for a lawyer is twofold - organization and discipline. Know ahead of time that 10-12 hour days and weekends are part of the package. On the other hand, don't get sucked into making your work life your LIFE. Get some downtime. Learn to say "no". The word "no" will be hard to get out of your mouth at times because you'll be convinced that your entire career depends on taking on more work than you can ever accomplish.
The lawyers I've met who regretted their decisions started out with the erroneous assumption they would work banker's hours and wear pretty suits.
I decided to become a lawyer because I topped out in my career financially and intellectually. I no longer wanted my hard work to stop at the printer. I wanted to "play" as it were. I also looked around at the comfortable lifestyles lawyers can achieve. Of course, the trade-off for that comfortable lifestyle is hard work - lots of it. But, if you enjoy the law, like making a difference, and like the law enough to eat chinese take-out at your desk, it's worth it.