I am an undergraduate student, applying for honours next year. I asked my current supervisor if he is willing to supervise me and he said he had to change the current project as I took a lot of time to do calculations, there was a lot of going back and forth and he said I am an average student and a project in this field in honours will be very challenging for me. I think that he does not have a good opinion of me. I am very heartbroken and have no idea what to do. Should I change my field? He said you should look for a field you are better at but I actually like my current field a lot.
I think you're too early in your career to say one way or the other. I had an undergrad prof say I should teach high school rather than pursue research based on my GPA halfway through undergrad, saying I'd never make it to grad school.
Now I'm a postdoc at a pretty nice institute in an extremely competitive field, and I've seen many other people who looked better on paper than I did at that point get weeded out of this career (not getting into grad school, failing prelim exams, not finishing the PhD, finishing the PhD with no postdoc offers, etc). Many people who have judged me as the stupider scientist and talked down to me couldn't make it in the field, so I wish I hadn't put so much weight on what they said at the time.
If you love the work, keep at it. Doing something you're passionate about for an undergrad degree is hardly a waste of time, even if you ultimately have to go a different route.
Keep a plan B in your back pocket. The nice thing about academia is the hiring timeline is usually a lot earlier than industry, so if you don't get into grad school/get the next level job, you still have a few months to pivot into an industry search. Depending on your financial situation, you may need to pursue both options simultaneously to avoid he risk of being unemployed, but it's doable. As long as you like the work/the lifestyle and you still have options, keep at it.
Also there are lots of professors out there with opinions, so don't just select one at random to decide your fate. Actually the bar is way lower: if you can find one person who believes in you at the undergrad level, that's enough, even if everyone else says you're no good. Later on in your career, shoot for at least three senior people who think you're great. Ignore the rest of the noise.
But an academic career can be hard at times, especially for the people that aren't flying through prestigious institutes with a ton of encouragement and no pushback. If you approach the career the way I'm describing, it will be really hard at times. If you find the work and lifestyle worth it, keep going, but the reality is that the lifestyle is not easy and there's no shame in leaving for a more comfortable life.