i think at this point you recognise that opening up fully is something you have to do, so there's nothing else but to bite the bullet and push yourself to share!

you can start with small things and slowly work your way up to full stories, but unless you find out the reason for your nervousness and solve it, no piece of advice will magically make you super open about your trauma.

it's always hard to step out of your comfort zone, but you know yourself that he's important to you and that he deserves to know whatever you tell him, so if sharing is really important to you, you need to open up even though you're nervous!

wishing you all the best :)