Hi,
First, I don't have the credentials to diagnose.
I am in research psychology, but this is not my direct research area, either. I just have done a lot of research and work in this arena as I grew up.
I know this is a bit late, but as someone who has been diagnosed with AD/HD combined type since the fourth grade and mood disorders from a very young age, I think that a mood disorder and /or AD/HD is a definite possibility. Especially the way you have described your struggles. Contrary to whatever your old therapist might have said, there IS such a thing as adult ADD. And if you had it, you probably had it when you were a kid, too (it is not like depression or anxiety, really. It doesn't just "pop-up" or "cure itself" with age). If you do have AD/HD, it is possible that it was very hard to get diagnosed because of your generally ability to succeed in academia. It is also possible that graduate school and its changes have caused it to be exacerbated. It sounds like you weren't prescribed the right meds at the right doses and you didn't have good therapeutic support outside of medication. And you were possibly misdiagnosed. Support and other therapy is crucial. Think of mental health like any other diagnosis. For example, if you have diabetes you can take insulin all you want, but if you don't monitor your sugar intake, you're still in trouble.
I'm not sure I would recommend something like a CT though. There is no one AD/HD test, and you can't really tell things like AD/HD or mood disorders from neuroimaging (though, there are patterns in the brain structure and activity of people who have them- it's just not really a diagnostic tool). In my experience, you should really be diagnosed by someone who specializes in AD/HD in adults. I have always been diagnosed with psychoeducational evaluations, but unless you want/need accommodations it might not be necessary. The disability services center on your campus can probably point you in the direction of a good resource, regardless- I would talk to them.
You might not have gotten the "high" depending on how you took it and your own biochemistry. I don't really get the study high from my meds, I get it from the AD/HD itself (hyperfocusing). AH/HD doesn't really mean that someone cannot attend, trust me. It is more of have trouble motivating, focusing and maintaining your attention appropriately. If you are having trouble doing that, you're likely to feel overwhelmed or bored, which can easily trigger anxiety and depression, which can exacerbate ADHD symptoms.
Just to go off on the "abuse/gateway drug" argument: (1) you shouldn't be penalized for someone else abusing the system and (2) Many ADHD meds are methylphenidate and (like someone else already said) they are all time released. Yes, it is similar to cocaine, but the crucial difference is how the drug is taken/released and how it increases dopamine. (Drugs of abuse cause a phasic increase in dopamine, most research concludes that the increase has to be phasic to lead to addiction. In contrast, orally taken whole methylphenidate pills cause an increase, a plataeu and then decrease in dopamine activity- it leaves your system and doesn't build a 'level') If you take the pill whole, you should not get the high you would with cocaine (that being said, it is definitely possible for it to produce physiological arousal. It's a stimulant). If you cut the pill in half, whatever was causing to drug to be time released is deteriorated- this is when you are more likely to get a stimulant like 'high'. It's science. (Still, if it's the wrong med for your system, you can get weird effects. When I went on adderall for a week as a kid it was like I was stoned. With I different drug, I can get through my day productively....usually. It is not one size fits all.)
I apologize if I rambled, but the the 'non-existance' argument for AD/HD is a pet peeve of mine, and I am glad you are seeing a different care provider. In an ideal situation, you'll be able to find a prescriber and a therapist (two different people) to work on both the chemical and cognitive side of whatever is hindering you. But you should know that you are not alone in your experience. If you find yourself diagnosed with AD/HD or a mood disorder (or anything at all/not at all) and want to talk to someone about making them work for you and not against you, feel free to message me.