I was in your same situation. I was a psychology major who loved the behavioral sciences.
Its worth it to go to medical school if
1) you are willing to deal with hard core sciences (not the social sciences--I'm talking the biological and physical). Most psychology majors I've seen cannot stand these science classes. In medschool, you will be given these types of classes on an order of 20-30 credits per term. Its not just a matter of tolerating these classes. You must master them, and at an seriously fast and difficult pace.
2) You are willing to have a very limited social life for years. Expect to have no life your first 2 years of medschool. My first 2 years of medical school, I pretty much only had half a day to myself a week, and that time had to be spent managing my life (e.g. paying bills, buying groceries, cleaing the apartment). The rest of the week, you will be studying at an incredibly difficult pace. You can have a semi life your last 2 years. Expect again to have no life during your first 2 years of residency. You might have a life your last 2 years depending on the residency.
On the other hand, most psychologists I've seen throughout their schooling had enough free time to have a social life of sorts.
3) You are willing to learn a medical-centric model of treating mental illness. You are not as likely to learn several behavioral and psychotherapeutic approaches as much as a psychologist would. Though some programs are exceptions to this rule.
4) Kinda on the same order as #3-You are willing to study incredibly large amounts of data that do not directly deal with mental health. While all fields of medicine converge at some points, and as doctors we have to know how to interact with the other fields, prepare to have to understand pancreatic cancer, diabetes, hypertension, myocardial infarctions etc whether you like it or not. Yes these do have important implications in mental health, however several psychology majors I've seen wouldn't touch this stuff with a 10 foot pole. The way it relates to mental health at least for me wasn't appreciated until years down the road because only then did I start to see how it all related. Its also not taught in a manner to entertain those who want to go into psychiatry. When medical professors teach about heart attacks, they teach it as if you're the guy who needs to treat it, not as the psychiatrist who has to keep it under consideration in relation to mental health.
5) You don't mind studying and working under extreme situations. You're hard core. Ok, yes I'm sure that happens too in psychology graduate programs, though I'm sure not every single one. Pretty much every single medstudent I've met has been through that "is it humanly possible to know this amount of information" situation--which presents different in all of us E.g. one starts taking the equivalent of 10 cups of coffee a day through caffeine pills to keep up, starts pulling all nighters 3-4x a week, gets a script of adderall even though they don't have ADHD, willing to stay up till 2am in the anatomy lab every weekend, etc. Plus you're going to have to deliver babies, do surgery, suture a wound, literally sawing a person in half with a sears craftsman saw using all your might for hours from their penile urethra all the way up to the top of the skull, being in an ER, and seeing some guy with bullet wounds literally dumped in front of the hospital by people who then quickly drive away. That's very hardcore vs studying from a book and interviewing someone.
I'm a first year medical student, and while my main interest is psychiatry, I also enjoy anatomy and most other things. If you have absolutely no excitement about biochemisty or learning how the human body works, it's going to be a rough couple of years.
Exactly where I was at. I knew I wanted psychiatry from the start. So why then did I have to memorize every freakin hole in the skull, or deliver 40 babies? Well I did.
Its one thing to tell someone to suffer a few months learning things that don't seem to correlate, but its another thing to have to go through years of it, at an intense pace, and then only after that long journey was crossed do you understand why a lot of it is relevant. When you do consult liason psychiatry so much of that medical knowledge does come into play. You may not see that now, but you will later on. It also becomes relevant in inpatient (e.g. you get a guy off the psychiatry unit because you discover the guy has brain cancer, and not psychosis.) When you prescribe meds to pregnant patients, that data from embryology starts coming into play.
I'm not going to lie and claim all of it was relevant--I still don't understand how memorizing how the histology of hair follicles exactly helps me treat someone with mental illness, and yes hair follicle histo questions did appear on my USMLE Step 1.....