I am one of those is often quite ambivalent and even negative about my experience in medicine and I'm very upfront in saying I wouldn't do it again. But this varies wildly based on individual circumstances as well. To the OP:
1. Age 23 is very, very, very young. I know it doesn't feel like it now, but it is. Trust me, when you look back in 10 years time, you will think you were a baby, because you are. Not to have income until you're 27... honestly, this isn't nearly as big of a deal in the grander scheme as you might think. A lot of people meet their partners in medical school, and you will be going to medical school in a big city where there are plenty of people to meet both in and out of school. Whether you are a boy or a girl, you will still be prime dating and childbearing age when medical school is done. You could even have a baby during or after medical school, while in residency.
2. I'm surprised you've had all these attendings, especially FM ones, tell you they were miserable. What would they rather be doing instead? Have they experienced other careers? We all suffer to some degree from a "grass is greener" on the other side mentality. People who work at "regular" jobs aren't always happy either where there's the real possibility of getting fired, having to compete for promotions, etc. You might hate your boss or coworkers, your work might be a long drive from where you (can afford to) live. There really are no guarantees.
3. If you are thinking about options, maybe you school would let you defer your decision for a year. However, I would think twice about doing this. One year goes by quickly, and you will have to make the same agonizing decision next year. It's a lot of time to be stuck in limbo, but might not be enough time to explore another viable career, before the decision comes up again, making things all the more muddled.
4. Many of my complaints about medicine stem from the fact that I had a viable career before medical school and only decided to go the medicine route in my late 20s. It made medicine a lot more difficult to bear - there was the opportunity cost of all the income I was giving up, having to move away from my friends, a lot of factors that contributed to my life feeling like a real funk. These things have gotten all the worse as I've matched to a 5-year residency in a location I don't want to be. If I knew things could end up so bad I would have taken the match a lot more seriously and put more effort into gunning for a good location. I'd feel a big sense of relief if I could go to a family program in the same city as my family - at this point I don't care about specialty anymore, I just want my life back and be close the ones I love. I also realize that a lot of my feelings could change later in residency or when I'm an attending.
5. You will never know in advance what's the right thing to do because life is a one shot deal and you don't get to know what would have happened if you had picked another option. Despite all the posts and blogs you read and all the people you talk to, it's still possible that you end up very happy in medicine and glad you picked this path. It's also possible that you leave medicine now and it later becomes a gnawing regret. Or maybe you leave medicine now and never look back. But what we do know is that even with a less-than-ideal career, you can still make your life very satisfying, both inside and outside of the career. So it may not matter which career pick so much as how well you deal with life's challenges as they come up and your ability/luck in finding a niche where you're happy.
And if you're someone who's prone to depression, you will be prone to depression no matter what you do. It's something that will stay by your side through thick and thin, ready to pounce. Sorry, that sounds terrible but as someone who's also prone to depression, that's my experience.