First of all, you have to consider your alternatives. If you went to the doctor for insomnia, you might get a prescription for a sedative-hypnotic like Ambien or Lunesta, both of which can cause significant memory problems (and even amnesia or blackouts). For anxiety, you'd likely be given an antidepressant (SSRI or tricyclic), as they don't usually prescribe benzos like Xanax to teens. Both antidepressants and benzos can cause problems with concentration, memory, and cognitive ability in some users, especially benzos. And if you tell your doctor you're having problems concentrating in school or your grades are slipping? The treatment for that is stimulants like Adderall, Ritalin, or Concerta, which can cause all manner of health/mental problems when started at an early age.
Smoking only moderately at night should not affect your grades in school. The effects of marijuana on short-term memory and other cognitive abilities are only present when the user is under the influence for the most part. Plus, these effects aren't unique to pot and can occur when under the influence of many doctor prescribed meds.
If you're having trouble in school, you should seek help. There's tutoring, one-on-one meetings with your teachers, better studying habits, and counseling if you feel your problems at school are being caused by something psychological or emotional (which is a real possibility with OCD/anxiety). I struggled with math all my life and was finally found (at grade 10) to have a learning disability in the subject. If it's serious, consider getting tested for a learning disability. If it turns out you have one, the school is required by law to tailor your assignments to meet your individual level of ability in the form of an IEP or 504.
If cannabis is helping treat your anxiety and OCD, using it responsibly and like any other medication (at the same time and in the same dose daily) should NOT cause any serious, long-term damage. In fact, it's a non-toxic substance with no known deaths attributed to it. You can't overdose from it, it's never been known to cause any specific diseases, and there is absolutely no conclusive evidence that it causes any kind of brain damage whatsoever. You seem to be genuinely using it in a responsible manner, so I'd say it's better than many of your alternatives despite its stigma as an illegal drug. The stress caused by OCD/anxiety is FAR more harmful to your body and mind than medicinal use of cannabis, not to mention the effects of stress-induced insomnia/sleep deprivation you use it to treat.
Of course, cannabis is not a long-term solution to these problems and should be accompanied by psychological support in the form of cognitive-behavioral therapy, counseling, or some other form of treatment. The same is true for SSRIs, anti-anxiety meds, and any other psychiatric medication. In fact, studies have shown medication alone to be virtually useless without some form of counseling, so keep this in mind. Ideally, adolescents shouldn't use marijuana or any other mind-altering substance--prescription or otherwise--because their brains are still developing, but in certain circumstances it's the least harmful alternative. The trick is in determining whether that's the case for you.
(Cannabis is commonly prescribed to adults and sometimes teens for mental illnesses like yours in the state of California.)
Sorry so long. I just hate to see people with legitimate medical needs so afraid to use cannabis because of all the "amotivational syndrome" "brain damage" propaganda that's burned into all of our psyches from the day we're born.
