When should you stop "doubling down"?
No one can answer this question but you. 

Having said that, some points to ponder: 


There exist funded research based masters programs. Depending on the field and geography they will vary substantially in quality, but conceivably you could delay answering that question by going somewhere for a Masters and seeing how well that goes. 
Grades are going to have an effect on you getting in. In terms of success, what really matters is grit. Smarts get you somewhere, but you need to learn to tolerate challenge and failure to succeed in research. 
If you have nothing else on your plate, in terms of family or ambitions, then now is the time to experiment with possible paths. So long as you don't define your self-worth based on success in a PhD, the worst that could happen is you decide research isn't for you. 
Is it research you want to do, or is research just the only entrance into a field you can see. Sometimes people think they want to do research, but it's because that is the only job they know in that field. Maybe spend some time thinking about alternative entrances into the field you're interested in!