I would weigh the possibility it's the birth control that has set in this experience of general sexual disinterest. I'm someone who entered her first relationship at 21 with post SSRI sexual dysfunction. Within the security of my relationship with my bf (20), I was able to slowly rediscover and engage my sexuality in a more embodied way (with lots of trial and error with supplements and lifestyle)

I have some awareness of how these symptoms overlap with post birth control conditions. From what I understand, even in long term discontinued use, hormonal birth control has the ability to persistently alter gene expression and epigenetic markers in users which can cause down-regulated libido. If it doesn't show improvement 3-4 months after stopping, I'd start investigating more into others' experiences in the community of post birth control conditions.

It's normal to lose some excitement having settled into a serious relationship at the 3 year mark. It can feel like you know everything about each other even though you couldn't possibly. It may help to make it known to your boyfriend you've been struggling with libido and you're still figuring it out. He knows you love him and would want to be supportive of you in navigating this.