Double check your horse to this checklist:
Horse is 2+ years old with 20% or more energy
Horse has its personality revealed (if not get it personality tested at the vet)
[This is the big one most people forget]
In the horses settings the 'Career' box is checked
In the horses settings the Career dropdown has a career type selected
Want to check out more about Careers and other pathways? Check out this guide page: Horse Pathways
We have an entire guide page just for this topic found here: [Guide] Making Bits
Different players will find different ways of making bits more useful / doable than others. The best thing to do is read the hyperlinked guide, play around with listed options, and figure out which one suits your playstyle the best! Of course, not all ways of making bits are listed out as some are a lot more highly promoted than others. For example selling horses is not listed on the guide, and a lot of people will struggle to make that a useful way of making bits. However, there are some players who do make a decent stash of their bits this way - you just won't see this as a promoted method when asking around though usually.
You can level up your player level and earn player experience through doing pop up quests found in the Hoofbeat Plains.
At this time you do not gain rewards or benefits from your player leveling up outside of bragging rights and possible placement in the Hall of Fame.