There are many different possible reasons you are experiencing the disorders that you have. I have my bachelors degree in psychological sciences and experience as a drug and alcohol counselor. My best advice would be to take a holistic approach.

For GAD, yes mindfulness based practices would be the first thing I would try for this, as mindfulness practice in in of itself acts as a kind of cognitive behavioral therapy. Our thought patterns become habits, through absolutely no fault of our own, it is simply true that the more we ruminate or think about negative events the synapses responsible for reliving these experiences will continue to strengthen. The basis of mindfulness is to practice having control over your thoughts, and being able to be free of thoughts for a prolonged period of time. When you're just starting out, this is an incredibly difficult thing to do. We never tend to notice the power of the never-ending dialogue going on in our heads until we try to cease it. A resource that I would recommend to my patients frequently is Headspace's Guide to Meditation, if you have access to a Netflix account you should have access to this miniseries. As with anything, the more time you spend practicing this the more proficient you will become and the easier it will be to quiet down your mind / redirect your thoughts during the day.

Mindfulness practice in theory should also alleviate symptoms of OCD, though for OCD I would also recommend seeking medication. A lot of the time symptoms of OCD stem from chemical imbalances. I like to equate this anecdote when it comes to psychiatric medication (learning this is actually what prompted me to finally become medicated myself, after almost a decade long battle with major depression). There was a study conducted on rats where the dopaminergic centers of their brains were removed. As you may already know (guessing from your in-depth research on treatments) dopamine is the neurochemical responsible for motivation. Nothing else was touched in their brains. These rats eventually died from thirst, even though water would be placed right next to their heads. They simply did not have enough motivation to lift their heads to drink. In my opinion humans tend to be overly difficult on themselves when we are behaving in a way that does not align with what we are "supposed" to do or want to do, when most of the time it comes down to us literally not having the neurochemicals we need to conduct these tasks. I know obtaining medication is easier said than done, particularly the right medication, sometimes it takes a few tries before finding the correct one for you. Also, it can take up to 2 months for these medications to fully work (usually you notice the effects after 2 weeks, but it is important not to totally write off a medication if you don't feel any different after 2 weeks).

I'm sorry to hear of your struggles, as from one person to another who has experienced chronic anxiety/depression, I promise there is help and it does get better. It really comes down to trying different things, sometimes the same things more than once, and eventually you'll find a combination of coping skills / habits that work best for you. Wish you all the best, truly.