Do I tell my boyfriend I messaged an old friend I used to have a crush on for closure?

TL;DR I messaged a past "what if" friend for closure after my therapist suggested it. We had a brief, healing exchange, and now I finally feel at peace. I am currently debating the best way to approach this with my boyfriend to maintain our transparency without causing unnecessary friction.

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2 years ago I (f23) was going through a really bad time. I was in a big friendship group but always felt out of place for personal reasons and drama I can’t be asked to explain.

During this time, this quiet guy (m22) in the group that always felt unapproachable and myself were left to hang out and we became good friends for a very short time after that.

He would protect me from a creepy guys, would always stand just too close, always stared at me, and stayed up until sunrise with me after nights out.

He was like a sanctuary to me at this dark point.

If I’m honest I was falling for him. It was cut short when I found out he had a girlfriend though, she followed me on stuff and made him unfollow me.

In real life he ghosted me as well. We haven’t talked since.

I’ve been really stressed lately and dreamt about him, I usually do when I’m stressed. He always made me feel safe.

I’ve never spoken about the things between us or our friendship to anyone. Nothing happened sexually, it was just memories that were precious and I kept private.

My therapist suggested after all this time I get in contact and thank him for being there for me.

I found his instagram and sent a message. I omitted a lot of my feelings from that time and the temptations to ask about stuff. I simply told him thank you for being kind, being a friend and just being there.

He replied a full paragraph within 1h, the boy who took days to reply once upon a time.

I’m sad that is the last time I’ll ever speak to him. But, I’m happy he was glad to hear from me and to know he made a big impact on my life in that short time.

A part of me will probably always wonder ‘what if’ in the quiet moments, but now I’m no longer wondering ‘when’.

I want desperately to be honest with my boyfriend (m22) and not hide it because he’s my soulmate. However, I only mentioned this boy in passing previously. I worry if I bring it up now, it will create confusion as to why I didn't mention the depth of the friendship sooner.

I am looking for advice on how to navigate this conversation with my boyfriend, or if this is something better left as a private therapeutic milestone.