First of all, I just want to say that I'm so sorry that all this has happened to you. With regards to Oxford rejection, I know exactly how you feel - one year ago, I was rejected from Oxford, the university of my dreams, and I thought my world had ended. I know that it was so hard to see others celebrating while all your aspirations have been shattered. I want you to know that it does, and will, get better. I know it probably doesn't feel like it (and if you told me that a year ago, I wouldn't have believed you either), but nevertheless your life will get better.
You said that taking a gap year is risky - maybe you could give that some thought if you are set on going to Oxford? In my experience, my Cambridge interviews this year were made a lot easier having already gone through the stressful Oxford ones last year. It all depends on subject though - I'm not trying to influence you in a certain direction, I just think you should know it is an option for you if you want.
Have you visited UCL and Bristol? I found last year that, having not visited St Andrews for example, I was pretty unwilling to give it a chance after being rejected by Oxford. However, I was much more open to the idea once I went to a subject open day and actually visited the place (if you have, ignore this haha).
With regards to your girlfriend breaking up with you, know that this seems to be more a sign of her unkindness than anything you've done wrong. As an outsider, it seems that her actions are quite childish and hurtful, and know that this isn't a reflection on you but on her that she decided to break up with you in this way.
Something I found helpful when revising last year was to uncouple my exams from getting into university - instead of using a university place as motivation, maybe you could think of the grades you would like independent of that? Also, I kindof tried to use my rejection as motivation (bear with me) when revising, as if I was 'proving them wrong' or something. Also, if you're worried about teachers judging your poor performance, they know that being rejected from somewhere like Oxford can be massive, and they'll understand if you underperform in your mocks.
Are there any other extracurricular activities that you could pick up to replace football for the time being? It might be useful to have something to take your mind off things.
Again I want to stress that you being rejected from Oxford doesn't mean that you aren't worthy, clever or talented - remember that they also have to look at whether you'd suit the teaching style at Oxford, and maybe the professors thought you would benefit from a different style of teaching elsewhere.
Edit: after reading some other comments, I would also recommend maybe talking to your doctor. As someone with experience of seeking help for mental health concerns, I can't promise you that there will be no waiting list and you'll get help immediately, but on balance it's better to try and access these kind of things than not.