I am sorry you had such a rough experience at your first birth. My experience was also not the best and i had flashbacks for months.

I had a student midwife follow me through my pregnancy and it was up to me whether i invited her to the birth. My intention was to invite her but due to personal circumstances at the last minute (a close friend lost her baby) i decided that i didn't want the student midwife to be there.

I went on to experience trauma during my labour and i kinda wonder whether having an independent person present may have changed this. However, i don't think my student was the right person for this role. As a student, at each appt she would ask the person i was seeing for a signature on her record of appointments. This was only a little thing, but to me it put her in debt to them and less likely to question them or support me in their presence to question them.

Post birth, i had a meeting at the hospital where i had made a formal complaint (re actions surrounding the birth) and was supported by an independent midwife. Wow, awesome woman who helped me feel strong to say what i wanted to say. She was an experienced IM but i think even as a student she would have been awesome.

So, i think it depends on the student and if they gel with you and are able to do what you want them to. AT the birth, you want to be comfortable and free to choose who shares your birth space. I would let any people know that i might change my mind and not call them or might ask them to leave if things weren't working but i would hope to know beforehand (through bulding a relationship during labour) who i wanted to be there