Do not be afraid of antidepressants, these are particularly helpful for anxiety and they don't have the drawbacks that benzodiazepines and other "downers" have in terms of impaired memory, learning and balance. For best results you might end up trying a few different dosages and/or medications, maybe seeing your doc every month for a few months, possibly making medication changes over the phone.
Meditation is good, but regular exercise should be the first line therapy for most mental health issues: a good goal would be to do 1 hour of cardio 5 days a week. It is good even if you do half of that, but it takes most people about 45 minutes to start experiencing the "Runner's High"
https://en.wikipedia.org/wiki/Neurobiological_effects_of_phy...
As for amphetamines, Ritalin, etc. I can say that I don't know any young adults who are taking those drugs to get into a high-functioning zone. I do know older (65+) people who use them to shake grogginess and I know children who seem to be doing OK.
I have been told, and I have learned, to give no quarter to negative thoughts such as you have described. It is not so easy to do because it so compelling, but trying to explore them in great detail tends to increase their power over you more than decrease it.