I'm thinking about keeping motivation and relationships.
Each term can bring up different topics. For example in motivation, there can be topics on: social anxiety, loneliness, financial stress, eating disorders, depression.
And relationships: body language, pickup, dating.
Since I'd like to treat both via Youtube, does it make sense to keep in the beginning only 2 main categories?
Describe your goal for categories, as a starting point.
Also answer where you'll define these categories.
You mention YouTube + you might mean content categories on a WordPress site.
If you're referring to a Website, also be sure to state whether you're primarily targeting natural search engine traffic or you're primarily using paid traffic.
Provide more context + likely someone can assist.
In the beginning, I see it as very valuable to keep the topics together. Especially if we are talking about YouTube and Website. The main reason for this is that you grow quicker if you have multiple topics - especially if those are related in some way.
Diversification will only lead you to have more work. More places to focus on. If one of the categories is more interesting in the long run, you can iterate and just change at that point.
If you want to discuss this further, please feel free to request a call. I am open to brainstorm and develop a future plan for you.
You asked your question some time ago but it's still open it seems so here is a pragmatic answer.
I don't know on what CMS your website is built, and I could give you a more precise answer once you have provided a bit more information. Perhaps your site runs on Wordpress?
You could have two "categories" and give the topics that you assign to those categories "tags" such as "anxiety" and "stress".
In php that would look like something like this:
query_posts( 'cat=motivation&tag=stress' );
If that does not mean anything to you, don't worry! I'll help you coding it and translate it into a nice "category chooser" that is afterwards filterable for the visitors of your website.
Feel free to plan a call, really five or ten minutes will be enough to understand your request and propose a pragmatic way forward!