There is nothing to forgive. You made a decision, which at the time you thought/believed to be the right choice. Whether it was or wasn't is neither here nor there. Your intention was right.
Obviously there's an awful lot going on. Maybe MIL could take the kids out (shops, park etc) to give you time to think through your options.
Maybe a mother and baby unit would be a good choice? You would still have dd with you, lots of hprofessional help and you know she's safe. Use MIL to your advantage. Let her look after ds if your dp can't. Ds will be safe, looked after and cared for.
Maybe MIL could commit to having the children set days per week? Like shared access? Would that ease the pressure on you and give you the time and ability to seek outpatient treatment?
Ultimately, you can't go on as you are. You need to pick up the phone and make an emergency appointment. Or walk in. Or call your health visitor/midwife and get them to do it for you. If all else fails, go to A&E (not ideal I know, but you can't continue like this).
You need to try to be open to changing the mess and trying the various treatments to find what works for you. It may take some time but you will find something that one day makes you feel slightly better. That's the beginning of the end, the light at the end of this really dark tunnel.
It's time to get tough on yourself...pick up the phone ir walk in the surgery. No excuses, just wait for MIL to arrive and do it. She will look after the kids. Maybe not as you would like, but they are her grandchildren and she will. Look at the longer term. You need to be here for them. You need to get better. You need to do this for them.