Magdalene:
I absolutely understand how hard it is as a Mum to see your daughter genuinely unhappy. I've been in a similar position this year myself (my DD also Y1).
Did you have a better relationship with the Class R teacher? It may be more effective to have a quiet chat with him/ her about how unhappy your daughter is and how concerned you are about her. It will get back to her current teacher or the head depending on the school politics.
It also may be an idea to suggest to your DD that she starts to play on her own if things are getting rough or upsetting. I did this with my daughter because playground banter was reducing her to tears and making her cry became something of a sport for her classmates. At first she really didn't want to be on her own, but she got used to it and now it doesn't matter if she plays with or without her classmates. It has turned out that there were several girls who didn't like the name calling and they've now formed a nice little group that supports each other. There are the occasional spats - but it's all much more calm now.
It's hard to get into 5/6 year old mindsets and they can't always explain their feelings or what is happening well - but children just want to be included and encouraged. Being the one left out or teased is incredibly hard. Encourage her to cultivate friends who are clam, supportive and generally well behaved. Gradually this will develop into stable school friendships and make life a bit easier. You can't control what happens at school but you can influence friendship formation through play dates/ outings & shared lessons (swimming/ ballet) at the weekend which strengthen bonds and lead to strong friendships. Also don't forget your nursery friends - my eldest had 2 dear friends from nursery and we regularly go on outings together, which helps to reinforce that friends don't have to be just from school.
In terms of finding Y1 a struggle - remember that a lot of the point of this year is starting the transition from learning through play to more formal learning. Sometimes teachers can be overly stern and this can be a huge shock after the cuddly/ happy teaching in nursery/ class R. But it is important to understand that they are trying to prepare the children for Y2 & beyond.
Yearning for things as they were before is partly about wanting things to be easy and happy again - your DD is really saying she doesn't feel in control in this situation and is finding the going a bit hard or confusing at times. Talk to other parents - you may well find your child isn't the only one struggling with the change (even this late in the year). There's no easy way to help her to cope with this, but celebrating the positive things about her day - a great painting, getting all her spelling words right, moving up a reading level, getting a merit point or a mention in assembly, etc... will help her feel her hard work is being noticed & celebrated at home, and reinforce that school isn't all bad. The trick is to make the good bits outway the difficult/ awful bits - that's a long project but slow and steady wins the race.
Hang in there and remind yourself and your DD that the school year is nearly over.