I don't know if this reply is a little bit late, but crazy things can happen man, I speak from personal experience. I'll cut a long story short:
A-levels were BDE in physics, chemistry and biology respectively. Even though I didn't get the BBB required, a redbrick uni in the UK accepted me onto their foundation year, making up for my lack of maths.
I coast through that, getting a 61% average mark, and enter onto the first year of a 4-year MPhys course.
1st year - averaged about 57% - didn't contribute to final class of degree.
2nd year - averaged about 63% - contributed 1/5 of the final class.
3rd year - averaged about 70% - contributed 2/5.
4th year - averaged about 73% - also 2/5.
Amazingly, I just manage a 70% average based on the 2nd, 3rd and 4th year performance, getting me "class one honours" - in other words pulling my degree (and entire CV) out of the fire with not a second to spare
However, during those last 3 years there were times when I got poor marks, for instance, in a 3rd year mathematical physics module, I got a 45 (scaled to 65), and a fourth year quantum module I made some stupid mistakes and ended up with about 57. Luckily, I pulled off some freak results in other modules (90's and high 80's etc). that evened things out.
The main contributor to having a strong 4th year was my big project, which got a hell of a lot of time spent on it, so I'd recommend not neglecting that. Obviously don't miss lectures, and really you just have to focus on hitting assignment deadlines, and reading enough of your lecture material rather than understanding nothing until the revision period (my worst habit). Also, in my experience, the best revision is just to practice the hell out of all the past papers and practice problems you have available - lecturers can be lazy... oh, and study with friends if possible.
So, my message is to not throw in the towel, because look what can happen. Unless, of course, this response is too late to matter. 75% is DEFINITELY attainable, and I say this as someone who averaged 73% in the 4th year despite giving into occasional ketamine and cannabis binges at the later (and most crucial!) stage of the year, due to mental exhaustion. Don't give up the fight man, you'll get it if you want it enough.
PS. It's also worth bearing in mind that the average mark alone is probably not the only thing they take into consideration when deciding your degree class. They'll look at any particularly high marks you have achieved, maybe even your attendance and dedication. You don't even have to get 70% to get a first, some people get it with 69 or 68. I am convinced you can do this.