I have OCD, as well as anxiety and depression, so I will try my best to describe what I feel the difference is for me.
For me, I have been diagnosed with anxiety and depression since I was 14/15 or so. But I was diagnosed with OCD more recently, because I suffer from a skin picking disorder (which is OCD related) and kept having episodes where I would end up "stuck" on certain thoughts or compulsions. I would pick at my skin for hours because I was freaking out about having skin parasites--even though there really was no logical reason to believe I had skin parasites. But the possibility that I COULD, even if I had no evidence, was enough to send me spiraling. This was definitely more OCD than general anxiety, and I think the difference is that while my general anxiety can still lead to getting "stuck" in thought spirals, it's not tied to specific compulsions to quell my anxiety. And in general, my anxiety is more fleeting whereas my OCD really feels like getting stuck in my thoughts or in an action I feel I have to do, and in general my OCD feels a lot more irrational.
My depression/anxiety has also been treatment resistant, but I feel this is more related to having developmental trauma. Given your mention of having an abusive mother, I wonder if its possible you might have some degree of developmental trauma (or C-PTSD)? Developmental trauma makes it less likely that antidepressant medications will be effective, for instance. If you're interested in learning more about developmental trauma I recommend The Body Keeps the Score by Bessel Van Der Kolk, which is a great book that discusses how developmental trauma effects individuals and complicates treatment. (but it IS treatable, just generally requires therapy and some other approaches)