As an undergraduate, is it alright to get help finishing my paper?

For scholarly work, as opposed to classwork, it is fine to ask for help whether you are an undergraduate or a full professor.

However, the nature of the help needs to be considered. For simple editing and phrasing, an acknowledgement of the help within the eventual paper is probably enough. But if it crosses the line to the point where the "helpers" are contributing ideas, then they become collaborators and co-authors.

Some publishers will provide technical copy-editors if that is all you need, but I think this is more likely for books than papers. It might even be that most of the editing you need can be done by a general text-editor as long as you vet what they suggest. Sometimes they will get it wrong and you need to be able to see and correct that. But such services are likely more available than someone with experience in your field.

After submission, some reviewers will suggest rephrasing as needed. But there is no guarantee of it. Telling an editor of a journal that you need a bit of help with the language might be worth the effort, but, again, no guarantees.

And, no, having collaborators and co-authors doesn't degrade the work. Collaborations with the right people can enhance it. But if you originated the key ideas you want to assure that you get credit for that. In some fields it is being the "first author". In others it is just a short paragraph describing contributions. But at such an early stage, any publication will help you along. There aren't a lot of people who achieve that.

Getting help is fine, and so is asking that someone collaborate with you. However, asking someone to collaborate is asking them to commit a lot of time. I can only keep at most three collaborations going at a time and am selective about this.

Asking the other mathematician to be a collaborator was perhaps a mistake. It seems like you have a result, and what you need is assistance with the writing. There are options other that asking "please read my paper" which are more likely to get a response.

Ask a professor in your math department if they know of a writing center on campus. Ask the people who ran the REU for advice on what journal to submit to. You probably need help with latex, selecting a good title, writing a good abstract.

There might be a seminar in your department where you can make a presentation. It really helps to have a forum to talk this over. Of course, seminars are not what they were in the Before Times.

Everything is slower and harder during the pandemic. This is why I suggest small things, like help with a good title. You may have "On an improvement to the glorious theorem of Dr. X" and it would take a professor five minutes to suggest "Improving the bounds in the theorem of Dr. X."

Best of luck. These are good problems to have.

You are in excellent company.

Nobel Prize winner Paul Dirac was famous for the difficulty he found in writing papers for publication where he would agonise interminably over phrasing. His colleagues in Cambridge jokingly defined a unit called a "dirac", which was one word per hour.

Ask for writing assistance and acknowledge it in your paper.

Good luck!