Will quitting a PhD to try an industry job severely reduce my chances of being accepted to another PhD program later? I know there are already a lot of "should I quit my PhD" questions on this site. But I have a specific dilemma  and I'd like to get some external thoughts.

After my Bachelor in mathematics, I didn't hesitate and started a PhD in pure math. After 7 months in the program, I'm having second thoughts. Research is not as enjoyable as I thought it would be. And I'm afraid that I might be missing out on other potential careers that would fit me better than academia.  

So I'm thinking of quitting my PhD, get a Master in applied math and try to get a job in the industry. I'm 20 years old, so I feel that this is the last moment to try another line of work without too many consequences.

However, if I do that but don't like working in industry, what are my chances of being accepted in another PhD program? I'm afraid that quitting a PhD once will severely reduce my chances. Are the graduate admission comitees likely to understand the "if you love it, let it go" flavor of my experience? Or will they dismiss my application thinking that they can't trust someone who has already quit once?

It may seem weird that I'm already thinking of coming back to academia while talking about quitting my PhD. The thing is that I have very little motivation to continue, and I feel like if I hang on, I will end up with a bad PhD after 5 miserable years. That's why I think of quitting. If after working in the industry I want to go back in academia, I think I will have a lot more motivation, knowing that there are no alternatives.
Congratulations!

Thinking of quitting or moving elsewhere is one of the important milestones in graduate studies.

We can't answer your question, as it depends on lots of personal and local particularities that we can't possibly know about. But one thing I can tell you: Don't take the decision lightly. Studies, particularly graduate studies, tend to stress you out, decisions taken under stress are very often regretted later. Wait until you have had some time to really relax (i.e., near ending term break, after taking holidays). Treat it as you would any other project evaluation (you had a class on that somewhere, right?): Set up your evaluation criteria, how to measure them (be it estimated time to graduation in years or personal satisfaction in a scale from 1 to 5), and define some weight for each. Then write down your value for each, and compare.

Never underestimate discussing this with your significant other, family, friends (on and off school), or even your advisor or other faculty.