Sorry to hear about your situation
. My MIL is similar...totally overbearing and a little looney I think thinking maybe she was the mom or something until we really set some serious boundaries with her. Do you have other options for childcare? Can you get state assistance? Are you happy with how she actually takes care of your son other than her relationship with you? Personally, I would have a talk with your boyfriend, try to get on the same page, and then both sit down and have a talk with her, tell her how much you appreciate her help (be sickeningly nice about it), but then firmly tell her that the things that cannot continue. And if your boyfriend won't get on the same page, I'd talk to her myself. This did seem to help with my MIL. She pretty much is clenching her jaw and will barely even look at me every time I see her, but at least she's not buying my kid things I said he couldn't have or questioning our parenting decisions 24/7.
As far as your therapist and psychiatrist go...I would suggest thinking about getting new ones if they're not helping you. I've worked as a social worker and have been in contact with countless providers who were basically useless. You mentioned anxiety...years ago before motherhood I was on Lexapro and it helped me dramatically! I am pretty sure you can use Lexapro while BFing. Also, it helps with depression as well. Don't stand by and let your psychiatrist offer you no help...find someone who will work with you on this. With almost every medication the question is if the benefits to you the mother outweigh any potential risks to your baby, and I am sure there are plenty of other options for you that wouldn't pose a huge risk to him. My understanding is the all the SSRI's pose similar risk (and very little....) but I could be wrong.
Hang in there...the first year of motherhood is definitely incredibly challenging and I think any mother would be lying if they said they never wished just a little they could return to their old lives. Definitely hard on relationships as well...just a little fact...couples report the very least satisfaction in their relationships when they have infant and toddler children compared with any other time in their lives. It gets better though. Good luck!