Can somebody suggest an online method of editing a self hosted jekyll site, similar to Prose.
What I would like then is the ability to add posts and edit content like a regular CMS. I am not interested in using GitPages.
Your question is off-topic
Please note that this is not a valid Stack Overflow question. This question will be closed because:
Questions asking us to recommend or find a book, tool, software library, tutorial or other off-site resource are off-topic for Stack Overflow as they tend to attract opinionated answers and spam. Instead, describe the problem and what has been done so far to solve it.
A question like this should be asked at Jekyll Talk, where the rules are less strict and moderators are easier on beginners. However, since you are a beginner at Stack Overflow and you probably mean well, I want to help you. I tried to answer your question as honest as I can below...
Answering your question
I have a lot of experience in building Jekyll websites with CMS systems. In my opinion (warning: opinion follows) there are two really good systems: CloudCannon.com and Forestry.io.
CloudCannon has a very fancy WYSIWYG editor, which most of my clients really love. Forestry does not have this, but has a little more functionality in their free tier. I really love CloudCannon, but I have to admit that Forestry.io is also very good.
Ofcourse other CMS systems exist, like Netlify CMS. Please do Google for them. You will defenitely find a whole bunch, but they will probably not be as polished, easy to use and/or affordable as these two.
Note that I am not affiliated with any of these systems (although I am featured as a Jekyll dev somewhere on the CloudCannon website).
Hope this helps you. If you have any other (on-topic) questions about Jekyll, the Stack Overflow community is more than happy to help. Happy coding!