Some difficulties in choosing a medical career? Let's start at the beginning. You'll need very good grades in school prior to admission to a college that has a pre-med curriculum. Advanced Placement and Honors classes are recommended. As an undergrad, you'll need general STEM classes, such as chemistry, biology and mathematics. This will likely be in the Liberal Arts college. Any physical sciences or engineering would be a good start, but definitely the first three I mentioned. And you'll need to do well in those. The difficulties are as with excelling in any studies -- study.
Eventually, you'll need to be admitted to a medical school. That's where your study background will receive much scrutiny. The workload will be significant. That is a real difficulty. The biggest hurdle there will be the stress of that.
Now, if you're referring also be eventually becoming a doctor, realize that you'll have to do your residency. This is an actual, practical use of your medical training.
Finally, a difficulty is how much time this will take. Undergrad studies will take 4-5 years. Medical school about the same. Residency will take 4-7 years additionally. And that's before you're eligible to be certified with a medical license.
Bear in mind that as you progress through these steps, you're learning valuable knowledge, so even if you decide along the way that this is not for you, you'll still benefit greatly.
It looks like you might be interested in a medical career, but I'm not sure if you had any specific one(s) in mind when you asked the question? I'm a current PA student learning to practice medicine, so I might be able to help you in some way, so let me know if you have questions about any particular job or profession.