Concentration is a skill which can be learned. Look up some self-help kind of sites about concentration.
You're 16. Relax a little. Isolation might very well be the single most common feeling among teens.
If you continue to have problems, revisit your therapist and look for underlying reasons besides ADD/ADHD which are troubling you.
I personally don't think Ritalin is a good idea for anyone - you're more likely to get symptomatic relief from cannabis than Ritalin (in my untrained and unmedical and don't listen to it opinion).
As much as it pains me to say it, she's right.
Hearing voices, visual hallucinations, urges to harm oneself, severe anxiety, euphoria, grandiosity, paranoid delusions, confusion, increased aggression and irritability.
Those are just some of the side-effects of Ritalin, it's probably not a great idea for someone that doesn't have ADD/ADHD that causes substantial problems.
For ADD/ADHD to be diagnosed, they must cause a real handicap in functioning in the home, at school, in the community, socially etc. And symptoms must be present before age 7.
I can relate to a lot of what you're going through. I have OCD, and while I'm not at all qualified to diagnose you, I will say that the way you think sounds very similar to the way I think. I get obsessive thoughts, and the more I try to push them out of my head, the stronger they persist and the more disturbing they become. My unwanted thoughts are sometimes violent, sometimes sexual in nature, or sometimes just plain gross. For instance I have an extreme aversion to vomit. If I think about someone vomiting, my immediate reaction is to try to put it out of my mind and try to think about something different. This makes the image replay over and over in my head, and it feels maddening.
The more you try to control a thought, the more out of control it will become. If you're telling yourself not to think about a pink elephant, you're still thinking about a pink elephant, and chances are you're thinking about it even harder.
You didn't make it clear in your post, by "inappropriate places," do you mean body parts like the breasts, buttocks, crotch, etc? Are you intentionally focusing your eyes on these parts, or are you just zoning out and staring in their general direction with your eyes unfocused? If you're making a conscious effort to look at someone, just try not to do that. If you catch yourself spacing out, try not to be so hard on yourself. I know what that's like too. I got made fun of a lot as a kid, so when people caught me looking in their direction, they'd accuse me of having a "staring problem." Because of that, I still struggle making eye contact as an adult. But I try to remember that everyone stares off into space from time to time. If someone confronts you about it, they're usually satisfied with the "Sorry, I just spaced out" explanation.
I wouldn't advise Ritalin since your psychiatrist has already ruled out ADD/ADHD. Meds alone won't help anyway. Meds can sometimes alleviate the symptoms, but not necessarily the causes of the symptoms. I'd advise you to see a therapist if you aren't already.
I don't know what to say about the concentration problems, but I do think you should try not to control your thoughts. Let them happen naturally without trying to stop them or change them. If your thoughts scare you, allow yourself to feel scared (this can be more difficult than it sounds!), but acknowledge that they are only thoughts, not necessarily reality. A lot of people with obsessive thoughts worry that their thoughts will manifest into reality. I'm not sure if you're one of these people, but try to remind yourself that as long as you can control your actions, you don't necessarily need to control your thoughts.