I can feel you. I used to have same issue like you, having lot of conflict with my in-laws. It was once so bad that I got post-natal depression and almost kill my firstborn. I use a pillow to cover her mouth in hope she can stop crying so that my pils will stop calling her "abnormal" and me "murderer" etc. It was lucky my husband was back home on time to stop me.
Firstly, I will say it is good that you are aware you need help. Second, you need a listening ear, someone who is professional or not does not matter but it must be someone who is mature enough to listen your issue to guide and be there for you at your lowest point. Third, start to love yourself more and give yourself small credit and start doing thing that make you happy. Fourth, don't blame yourself over the kid, I am sure you love your kid like I do just that we are overcome by anger, sadness or whatever feelings that make us lost our cool at that moment.
It took me 4 whole years to learn to understand and let go of my past. It take time for you too, be it baby steps don't lose hope and start to look for one you can trust to talk or even those helpline.
Hope my experience does help you a bit.