I don't think I've ever posted in this forum before, but I've been going over the same situation all day today in my mind, and I feel like I need to get it out of my head. Any advice or words of support would be really appreciated. I am feeling really upset today and I need to gain some perspective

I just started working for an agency that provides mental health support shifts, so that I can get experience in the field to start my career (hopefully!) in psychotherapy. Yesterday when I was placed in a high security day centre for kids with severe learning disabilities. I'm naturally really nervous about the work, how varied it will be and the different people I'll meet each day. To cut a long story short, I arrived at the centre yesterday morning to a fairly unwelcoming and disorganised start to the day. The manager told me to put my bag away and meet them downstairs, but forgot to tell me the door codes, so I was stuck in reception until he realised I hadn't met them, and came to find me. When I entered the room, which was full of people who knew each other, and a really young and distressed screaming & crying girl, I wasn't introduced or acknowledged by anyone else there. I waited until I was finally acknowledged, and then the other staff continued their conversation whilst the girl remained crying, screaming and bashing her head in the corner of the room. I had to enquire for myself what was upsetting the girl, whether she was normally in this state, and also what to expect for the entire day. I wasn't really getting any information at all, so I just sat down with the staff and remained quiet until someone addressed me.
The morning unfolded and gradually other children came into the centre, each with different needs. After being given no direction, I paid most attention to the screaming girl who seemed like she required considerable support. As the morning progressed she became more and more agitated, started trying to run around, taking her underwear off and so on, and I was there looking after her.
At one point, when she was playing quietly, I went to sit down at the table with the rest of the staff (still facing and close to the girl). My back was turned for ten seconds when all of a sudden one of the fellow support workers burst through the door and screamed at me 'CAN'T YOU SEE WHAT SHE'S DOING!?' and pointed at her whilst staring me furiously in the face. The girl, having escaped my attention for a split second, had found a broken piece of metal and was playing with it. I was really upset by this and felt humiliated as she demanded I go over to the girl and 'fix' the situation with her shouting at me and watching me. For starters I wasn't told that I was supporting her one-on-one, and secondly the woman who shouted at me was in the exact same job role as I - she could have easily taken the metal from the girl herself and not shouted in my face.
Some time passed after this, all the while I was quietly really, really upset and embarrassed, and I had to take the girl into a sensory room to calm her down. When in there she started screaming again, and I had to bring her back into the room. She didn't come willingly, and I had to drag her - all the while unsure whether this was actually allowed?! I knocked on the door - as I of course was not told the code to get back in - and the same thuggish woman shouted 'WHAT, YOU CAN'T CONTROL HER?!' at me, and grabbed her in and pulled her back into the room.
I spent the whole day really upset and trying to regulate my emotions inwardly and dreading any interaction with another person. I went to bed at 7.30 and slept for 12 hours because I was so exhausted from the day, and upon waking today the first thing I felt was depressed and anxious about how I was treated by the staff yesterday, at a new workplace where I wasn't even sure of what I was supposed to be doing.
Now I'm just really doubting myself as a person. I feel really weak and like I have no authority or backbone at all... like I shouldn't be in any position to support a vulnerable person. I am trying so hard to be strong and assertive but I'm a very introverted person. I can't just walk into a room and exude energy, if I try I come across as trying to hard, and I'm much more comfortable just sitting quietly.
Any advice on dealing with people like this in the work place would be much appreciated

Also any advice on being assertive or more confident in myself would be really appreciated






