Tell me, did you fall for a shooting star?

Show your character's struggles with holding in emotions. Just because your character won't let themself go in public, doesn't mean they don't struggle holding everything in.

Find a motive. Pride and/or consideration for those around them can make emotionally reserved people hold back more than would be considered good for them. There are other reasons too, perhaps the initial cause and the reason now are different, but try to give them a reason.

Show side-effects. Everything you bottled up shows itself in another way, whether it's physical or mental. This can be long- and short-term, depending on the severity of the emotions.

Give them coping mechanisms. Everyone who wishes not to show certain emotions has a trick or two to keep themselves in check. Taking a sip of water, not looking people in the eyes, clenching their fists, breathing just a bit too deeply.

Who is this new version of you?

How do they dress?

What do they think about?

What do they enjoy doing in their spare time?