When was the last time you felt guilt about something, as in that emotional pit you noticeably feel in your stomach when you have done something wrong towards someone?
Forgot my best friend's birthday. We don't normally get each other anything, or celebrate in any way, so it slipped my mind. Felt awful and wanted to turn back time so bad.
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.