How to support partner w/ CPTSD?

my partner has been going through something (maybe past traumas being brought up) which they have not spoken to me about but i can tell they’re suffering alone. Due to overthinking and built up thoughts, they decided they do not want to date at the moment. I know them and I am 99% sure this is due to overthinking and not feeling like they’re good enough for me. How can i support them and help them open up? I need to get things off my chest and i’m sure they do too. I wish they would tell me more and not hold everything in. I’ve been trying to educate myself more on how I can support them through this. We’re each others strongest supports.