What helps me is to take it slow. We are used to modern language where we can read a text very quickly, sometimes even skim it, and still understand the gist of a text. With older texts, that won't be your experience at first. Go through very slowly. Like u/mayaxs suggested, stop regularly and decide in your own words what the text was saying. That may also help you remember what is being said.

If you get overwhelmed, take a short break. Pushing through when you're stressing may make you even less likely to understand. Go take a walk, play a video game, whatever clears your head, then come back to the text.

I also find that having the context of a class helps a lot. Let's say the professor has been talking about (or will be talking about, according to your syllabus) nobility in Renaissance Italy. Now you know what you're learning about! Now you can look at your document knowing that. What's going on in your document? How does it tie in to the course? What can it tell you about the topic? I find that reading with those sorts of things in mind makes it way easier to me than if I'm just blindly reading a document with no idea what it is or what to look for.

And of course, if all else fails, talk to your prof. Seriously. It's their job, and many of them like when students come to them to honestly seek improvement.

Have you reached out to your professor? They should have designated office hours for you to stop in and ask for some guidance and help. You can schedule a meeting too if you prefer. Since your professor is the expert in this topic and is likely quite used to navigating these difficult-to-understand texts, they should be able to give you pointers on how to approach them. Seeking some help can go a long way! And I hope someone here provides good advice on how to approach these readings as well!

I abide by the 2 sentence rule. After each page of prose, write a 2 sentence summary of the page. While note taking can be helpful, the thought process of summarizing it in a limited space helps distill the information (at least for me) to its most important parts, and with any luck (assuming your professor is not a complete dick), that is what the reading quizzes will be about. EDITED TO ADD: Also make family trees! They’re extremely helpful when defining relationships between characters, and help with repetitive names (if I had to differentiate every þórdís vs þorvaldr in The Saga of the Icelanders, I would be totally lost)

Oftentimes trying to focus on understanding every word will be slow and frustrating. Try reading a paragraph at a time and try to get the gist of each paragraph. By reading in chunks like that, you can gain more context even if you don't understand every word. Also, pay attention to the kinds of questions being asked on your quizzes to kind of filter less important information.

Slightly late to the party, but: I'm not sure how "old" the "intense, old language" you're struggling with is, but if it's something like Middle English where you're dealing with a lot of unfamiliar spellings, try reading the passages aloud. That helped a lot when I was studying Chaucer, especially at first. Words that didn't click when I saw them written often would make sense once I spoke them.