PPD, Intrusive thoughts, Self Harm

I have PPD, which I'm currently taking 25 mg of Zoloft for daily. I still breastfeed my son, so I have to take what's ok for breastfeeding. From the outside looking in I seem to be dealing with it better. I broke down at the end of June and sought help, which is great and all, but my therapist and the meds only help so much. I still have thoughts of hurting myself. I used to constantly have thoughts of hurting or abandoning my son, usually along the lines of "what if I left him in that dumpster" if we passed a dumpster while on a walk, or "what if I were to just bash his head in with this". Usually these thoughts are accompanied by VIVID imagery of my son mutilated in some way or deceased. Thankfully, the thoughts about my son have almost completely gone. In their place though, is self harm. Thoughts of cutting myself have been consuming me. Three showers ago I held my shaving razor to my skin, but fought against myself and the urge to cut. One shower later, instead of using a 3 bladed razor I used a small pair of scissors. The scissors worked, but were dull and it took a bit of effort to cut. I thought if I just held them against my skin that I could convince myswlf not to use them, and that I coukd overcome the urge. I didn't. I used those tiny scissors that I usually trim my eyebrows with as a blade, and I cut myself on my thigh, high enough where shorts will cover the cuts. It was hard to cut with the dull blade, but that didn't make it any less painful, I feel like it may have been a little worse because I had to go over the same spots a few times to make it actually cut. I didn't bleed, but one cut I obviously broke the skin enough because it has a scab over it now. The last shower I took I did it again, but less lines and a tiny bit less deep. The thing is, I don't know why I have these urges to self harm, and for some reason afterwards I felt a little better - like I let out some of my shitty feelings. I'm worried this will become a habit and I don't want to be committed to a mental health institute, as I have to take care of and nurse my son still. I am worried though that once my son is weaned, that I'll end up feeling like he won't need me anymore and that I'll become suicidal. I already have suicidal thoughts daily and I can't get out of my own head. My head constantly tells me that I'm a shitty mom, I could be a better mom, that I don't enjoy spending time with my baby as much as other moms do, that the longer I stick around the more I'll end up hurting him in the end, and so much other shit along the lines of "just run out into traffic" "drive off the bridge" "crash the car". But I want to see my son grow up and I want to be the best mommy I can be... It's just that my head is against me...