How do I degloss my shiny bike?
If you really want to paint it, you can try to wetsand it to remove the clear coat, and then refinish it with some matt finish clear coat. This is a risky move if you're not experienced with painting cars or stuff. You can ruin your bike real quick.

The safe method, and easiest one, would be to wrap the bike with some matt/satin finish clear vinyl (here is an example). It is often used on cars by people with the same issue as yours, and would perfectly do the trick on your bike. Plus you can remove it whenever you want, and it will protect your frame. It's a win-win.