What are some disadvantages of becoming a doctor? What are some specific struggles that come with being a doctor?

There are so many different ways to answer this question, and I'm hoping that professionals who directly work in the medical field will chime in. The biggest challenge that I see many doctors face today is the amount of money it costs to go to medical school and (as a result) the huge amount of debt that new doctors face when they graduate. Keep in mind that after medical school, you'll typically have 4 or more years of working as an intern and resident to gain experience where you receive an extremely small salary.

One way to get around this might be to consider joining the military. All of the branches of the military offer scholarship programs for various types of doctors, and this can be a great way to not only graduate debt-free, but to also earn a nice paycheck while you are in medical school. The military is certainly not for everyone, but if debt is a concern, it is certainly an option worth considering.

That's a tough question to answer because I don't want to focus only on the negatives but that's what you are asking so here goes...
The disadvantages would include medical school debt followed by several years of having a low salary as a resident, and starting your "real" job later in life than other fields, so this cuts into your retirement savings. This is somewhat balanced out by having a relatively high salary once you do start working.
Another is that due to how the health care system and insurance company system is structured in the US, doctors cannot often spend as much time as they would like with patients, and order whatever test or treatment they want anymore. You have to get authorization from insurance companies to order expensive tests, and you have to spend a lot of time documenting things so the insurance companies will pay for the service you are providing. The costs of running a practice can be high and reimbursement in many fields can be low. This is why many doctors are not doing private practice any more, but are employed by hospitals.
Being a doctor is a real full time job, in that your hours will usually be more than 40/week like other jobs, and you will have to sometimes go take care of a sick patient instead of making it to a family or social event. It takes some creative juggling and getting help from others to balance your family life and work. But there are many different types of jobs within medicine and there are people that have easier hours or part time jobs as well.

That being said, there are many many advantages to being a doctor, especially having the ability to make a huge difference in the lives of many patients, along with being able to continue learning new things and teaching the new generation of doctors. So if you love your profession, the things that I mentioned above are balanced out by the many positives of the work.