a university graduate and normal shop jobs

is it hard to get a job at asda or something like that? will they not take you on because they see you as overqualified and will not be with them for long

It's not hard to get a normal job at all.
There is no such thing as being overqualified - they take anyone who can talk, read, follow rules, turn up on time and not make problems to the business or the managers, that's it.
All they care about is:

oh ok, it's just that I have applied to so many and I have always been rejected so my mum said it is probably because i am overqualified and they do not want to invest in me because I will be leaving when I find a graduate job

That could be a reason too. Not that you are overqualified, but the fact that you may quit once you find a graduate job.
Although I wouldn't put that as a top reason for that.
Have you tried walking in and just asking if they are looking for workers? Many times myself and my friends would get tons of rejections online, but then would easily find a job just talking to a supervisor.
For example once when I was at local sainsburys picking food I asked if they were hiring as my friend was looking for a job so I thought I'd ask and they became so excited they almost wouldn't let me out without promising them I'd go and apply for the job right there and then!
I think the reason you get rejections could also be how poor their digital recruiting systems are and many times the hiring manager just backtracks the candidate through the system when they find a good one.
All the best!