The thing about high school is that it's not the subject material that really matters, it's your ability to understand how you learn and develop the discipline to learn it. Even people who completely coasted through high school often get slammed in their first year of college because all of the classes are much more difficult than they were in high school (and many people were also used to putting in 0 effort for studying/learning in high school while still doing well). They then mistakenly believe that this means they are not good enough for college or not cut out for pursuing their dream jobs etc. Don't worry about it too much, as feeling that way happens to a lot of people when they start college.
Take the time to figure out what works for you and stick to that. I know for me my biggest issue was discipline - I hated studying, I hated sitting down to write papers ahead of the deadline, I hated any kind of responsible approach to my schoolwork. However, the schoolwork was important and my need to be a procrastinator was not, and I had to change my learning habits to accommodate that. Yes, there are plenty of classes that are pretty hard in college, and yes, in some cases paying more attention in high school could have given you a (very small) leg up in that class. However, doing poorly in high school is not even remotely close to being the nail in your educational coffin. What's important in college is being able to advocate for yourself and your needs (do you need a tutor? testing accommodations? extra help from the professor?) and take the time to get to where you want to be.
As you've seen from some other posters, high school really doesn't have much impact in terms of actual vet school admissions. However, I know that anxiety about not being good enough is something that can definitely impact your ability to do well in your classes, and I encourage you to take some time processing that and cultivating a mindset that's more realistic about your own abilities and capabilities. Most people are significantly more hard on themselves than anyone else is on them and you should know that doing poorly in high school has nothing to do with your ability to succeed in undergrad.