You just gotta prepare something for their birthday next year. No need for something big, just thoughtful. Add a sincere apology or two and I’m sure they will appreciate it.
I also tend to forget someone’s birthday other than my partner and immediate family so I just put them in my calendar. No more accidentally forget about friends’ birthday, unless I’m really busy and my ADHD acting up.
Ghosting someone (multiple really) when I just couldn’t handle all the things they expect me to do. It’s childish and very much my own mistake for not being clear with my boundary and overestimating my mental energy.
Slowly making it up one person to the next.