Ardha:
DD1 is at a comprehensive and has experienced similar. She also has 3 good friends from primary at a local grammar school and they all have had a bit of a shaky start.
several things - feeling bottom of the class/ feeling alone/ feeling they don't fit in/ feeling overwhelmed or confused/ feeling tired.
I think the pace of grammar school classes can be unrelenting and if you don't keep on top of your work you can feel you're a bit behind. It's also an atmosphere where the priority is highest achievement - so they want swift progress and kids to just 'get it' and move on.
I know that a work colleague's son has found the commuting exhausting - leaving at 7:30 a.m. and returning ~5 or later if he has clubs. Long days and leaving/ coming home in the dark can be very wearing.
January is quite a slump after the excitement of a new school in September, settling in October and then looking forward to Christmas. Suddenly in January you can find it's back to the slog with the next long break quite a long way away.
Socially if you are the only one from your primary or not with your other friends it can seem isolating. There's a tendency to see older kids or kids from your ear chatting and feel they're happy, having fun and you're left out - odds are they also have their worries/ stresses - they just have the advantage of already knowing people from their schools, clubs, tutor groups, etc....
Try and arrange to see old school friends at half-term or keep in touch (texting/ imessage/ ye olde telephone call etc....).
I think one way to resolve loneliness is to join a club or group - so that one or two days a week you're doing something you enjoy and interacting with other kids on a non-academic level. Just having fun and maybe making a few friends.
In terms of work and feeling behind - being honest with yourself - I don't get this maths concept or science lesson/ etc... and using the internet, older siblings, parents, etc... to help you understand better is key. Suffering in silence and not doing a lot to help yourself understand/ catch up just exacerbates the problem.
HTH