how do i get a job during my gap year

I started looking for jobs since my last alevel exam, but the job search is really demotivating me I knew I would get rejected numerous times but it's actually a bit outrageous now. I've mainly just been applying for jobs that don't require experience since I don't have any such as store clerks in aldi, lidl, tesco etc. whilst also applying for hospitality roles such as waitressing basically taking everything I can possibly get - beggars can't be choosers - and better yet there's been such a small number of vacancies for these roles, literally when I tell you there wasn't one entry level job for sainsburys within 10 miles of my home. Additionally I'm finding it ludicrous that I'm being asked to write a cover letter and a CV and do an online personality screening for 30 minutes just to get rejected each time! and on top of all of that it's for a minimum wage job stocking shelves. I understand these jobs can be difficult and I respect these workers but I can't fathom how hard it is to get a job as an inexperienced 18 year old having just left school, so it's not that I don't have a reason for being unemployed. I've put down volunteering opportunities I've done in the past but to no avail, had adults check over my CV and they say it's good and up to standard so I have no idea what to do.

I feel your pain. I don't sympathise though; it's like that for everyone.
Typical rule of thumb: out of 100 applications, you get about 5 interviews, for 1 job offer. First question is have you been applying enough? As a rule of thumb, the numbers can be higher depending on the role, location, industry, etc.
Second, have you done any networking? Networking your way in is usually a lot easier than applying directly. This is particularly the case for say supermarket and retail roles, where it's more about who you know.
Third, have you applied to small companies? The medium and large companies tend to get a lot of applications. They're essentially the places where everybody applies to. The smaller companies tend to get overlooked.
The thing with larger companies is that the hiring process tends to be more bureaucratic, and if your application is at the bottom of the pile, they might not even get to it before your gap year ends.
Fourth, what professional qualifications do you have? It's fine and dandy to have GCSEs and A Levels, but they tend not to be relevant for work. Yes, you know how to recite Hamlet, differentiate complicated equations, and put together a concoction that can blow up the lab, but what does any of that have to do with working in a restaurant?
If the job requires specific professional qualifications (e.g. Level 3 PT qualification to work as a PT, CSCS card for construction work, hygiene certificate to work in kitchens), then it makes sense to do them prior to applying. If you don't know, then look up the job profiles for them (ask if you don't know where to look).
Fifth, employers do look at your personality when hiring you. It's unfair, borderline discriminatory, and really really unethical, but it's partly how the companies work. If you don't have the personality for the role, expect to make a poor performance and therefore a short tenure.
Sixth, whilst volunteering is nice, unless you intend to work in the charity sector they tend not to add much. Employers are looking for paid relevant experience that closely matches the role you are applying for. If it's not a full salaried role doing similar things that you are applying for, it's not likely to mean much.
Seventh, unless the people you show your CV to for feedback are the same people who would be on the other side of the hiring desk of the company you intend to work for, getting their feedback means very little. A good CV by definition means different things to different people for different roles at different companies. I can show a CV to one person who thinks it's a masterpiece, and to another who thinks it's less than scrap paper. CV writing is an art form, and it should be tailored to the person you are marketing yourself to, otherwise it's not much better than offline spamming.
Eighth, if you did apply speculatively, do chase up with things in at most 2 weeks - ideally 1 week if they are not flooded with CVs. Sometimes your CV can get lost in their to do list (hiring people is not always a priority). If they don't have any vacancies, you can ask whether it's OK if you check back on them in say 3 months.
Ninth, I get a little annoyed when people who are intending on going for a gap year to only be focused on jobs that everybody applies for and requires next to no qualifications. If you are willing to broaden your horizons a bit, you can try applying for jobs that require completion of a qualification (that you can complete in a few weeks or less) or are not the standard: catering/hospitality, supermarket, care home, retail. There are roughly 600 different jobs that you can do with qualifications up to A Level standard. Have you looked at the others?
For example: civil service, admin, IT support, call centre, paralegal, life guard, PT, teaching English as a foreign/second language, financial advisory? What about working abroad?
So long the qualifications are NVQ Level 3 or below, it shouldn't affect your funding for a degree (even though you might have to pay for the qualifications yourself).
Tenth, what sort of career or job you intend to get after uni? It probably make more sense to do something that's more in line with what you want to later do. For example, if you want to become a solicitor, you might want to look for jobs as a paralegal; junior marketing roles for marketing; teaching assistant roles for teaching; nursery assistant roles for reception teachers; property for quantity surveyor, etc. Although you will quickly notice that you can get into a number of these jobs without a degree, you can sometimes get the option of doing the degree instead of an apprenticeship.
Do note, if it's in a competitive role, and internship or relevant work experience can really help with future applications e.g. if you want to become a doctor, work experience in hospitals is recommended; as with vets in vet practices, dentists in dentistry practices, finance roles in the financial sector.
Without knowing the specifics, it's a little difficult to help further. If you have specific questions that you think I might be able to help with, then let me know.

Firstly I wanted to thank you for taking the time to write such an in depth response, and I hear you when you say everyone goes through the same thing.Since I've been applying to so many general jobs it's hard to come up with a specific question I suppose there's just so many factors that go into it and it started to feel very messy and demotivating but genuinely thank you for writing this up I appreciate the help