Ok, well, first things first. You don't have to go to it.
But, not many people do that. And if you do go, somehow, someway, you will get through it. Even if it's in a fuzz of tears. Everyone there will understand.
At my mom's funeral, I had shingles. I also read the eulogy and carried the coffin. I didn't sleep a wink the night before. I was in agony. But, I did get through it. And everyone was so lovely to me. It really helped actually.
When my friend's mum died, she had to have diazepam to get her through it.
Just take it slowly, try and stay in the now. Think about related things, like what you'll wear, timings, but don't think too hard about the actual funeral and trust in the process. We do these things for a reason.
Have you got an support in real life?