I can only speak to science PhDs.

If you're not academically capable or can't find the right mindset, then being in graduate school won't end well. You need to be able to perform well intellectually, psychologically and spiritually to get into graduate school because graduate school requires everything the process to get in requires except substantially more. If you can't do it now, you won't be able to do it later. If you don't want to do it now, you don't have the personality to do it later. If you need to, take some time off and go back and kill it. If getting "A" grades is genuinely hard for you in more than a select few courses, you're not cut out for graduate school. That insanely hard exam in that one undergraduate course is nothing compared to the rigors of graduate school, such as being 2 years in with no data, a PI who's suddenly unable to afford you and a failure at first pass at candidacy exams. This is the reality, so embrace it. Recognize your ability and be honest about where you are and what you can do.

What makes a strong science graduate school applicant (I can ONLY speak for PhD programs) is research experience. There are so many naive students who know nothing about the reality of research and who have these delusional plans for after their graduation with no backups. You should realize that funding is abysmal. There's more bad science than good. There's more drama in big labs than there is in high school. Wages for PhD holders largely suck. Professorship is for a select few, and most PhD holders end up part-time lecturers making almost minimum wage per hour of work done. A career in industry is volatile and not for most people. Pedigree matters, A LOT. You'll likely never have your own lab, unless you're part of the select few (which everyone thinks they will be).

Research experience shows that you've experienced this. That you've spoken to the post-doc in their late 30s on an NIH pay scale (which is abysmal). That you've had every experiment never work all the time. That you are capable of thinking about science in the context of the research climate (publish, publish and publish). That you're able to design experiments and interact with other scientists.

I don't want to dissuade you, but this is what you'd hear in a real lab. You can either do it or you can't. If you can't, you won't get in. So if you know research is for you and you can do it, just get better grades. Getting mostly "A" grades shouldn't be mission impossible. If it is, reconsider another 6-10 years being a student, because it doesn't get easier, ever. Get research experience. If you get better grades, get research experience and show you're able to think about research maturely, you'll get in somewhere.