I'm sorry you had a bad first experience here, but you run the risk of bad clients regardless of how you get them. Maybe you were thinking that there was some vetting process on Upwork, or only great clients use Upwork. Upwork is just a place were people can post jobs they need done and people who want to do those jobs can offer up their skills. It's up to you to manage your client. It is a learning curve. Just because you've never had a bad client through other resources doesn't mean they don't exist. I've found my absolute best clients through Upwork. I am extremely picky about who I will work with. If I don't feel like I have a connection with a client right away or I feel like they are going to be more trouble than they are worth, I don't work with them. There's no reason to work with a bad client. The only bad experience I've had here is when the client brought in a project manager after I was half way through a job. Had I talked to the project manager before starting the project, I NEVER would have taken on the job. My point is that you have to be the one to figure out if you want to work with a client. You are in charge of who you work with here. No one is forcing you to keep working with that person. And there are bad clients in all walks of life. Just be more diligent next time about interviewing your client before you take on the job so you know what you are getting yourself into and you know if you are the right person to help that client. There's no reason to blame Upwork for a bad client. The more you do it, the better you get at it.
It's very rare that a newbie gets invitations. The invitations they get are usually scams. Also, to work for a client that says she will not pay you if she doesn't like yoiur work is such a red flag. You won't find any long time, established freelancers even bothering to read beyond that statement.