You cannot be with your ex girlfriend. I know it hurts but you cannot be a family together the three of you.
If she tried to stab you and has lied to you then it doesn't like she treated you very well.
You need to prove to SS that you are over her and just want what is best for your son.
SS have decided that you and her together are not good for your son.
They will not change their mind about this. You have to stop saying that you want to get back together with her. This makes it look like you are not listening to them.
Your son will not grow up with the three of you as a family unit. That is to protect him from violence and arguments. He needs to be around adults that are calm and caring not shouting and fighting.
You have done some good things, especially with your volunteering etc. and it is clear that you love your son.
If you are unable to sleep etc it may be a good idea to speak to your doctor.
Please forget about any relationship with your ex and do everything that SS ask you. Like other people have said, keep a note of everything you are doing to improve your life with dates etc and email social workers every time you speak to them to sum up what has been said.
Keep focussing on putting your sons best interests first. Good luck