First time self harm

I am really ashamed and confused. I'm going to be 47 and for the first time last night, I self-harmed. I've been reading up on it because honestly I know nothing about it or why I did it. But what I'm reading seems to makes sense. I went through a very difficult couple of years where everything that could happen did and it ended with a bad break up. I'm currently being treated for depression and take Lexapro. Felt like I was really starting to level off and then just out of the blue, for no reason, I did it. True, this followed on the heels of seeing my X the day before and it was very emotional (but ended civilly and I felt like we could both finally move on.) So maybe that was the trigger.

I might have done something foolish once or twice when I was younger, but it wasn't with intent to harm I think.

But now, I don't let wounds heal that I have. I bit skin on my finger down to blood (which I've always done during stressed periods through life), but last night was different. I started with a dull knife on my wrist to see what it felt like, and then incrementally moved up to sharper tools. Just scratches to draw blood mostly. Then I moved to my inner thigh. I really didn't think it was anything, until I was walking around and I felt my white boxers were wet and looked down and the area was soaked with blood. I was just amazed because I really didn't feel much when I cut, but there was a lot of blood. It was very late, so I made sure to carefully clean everything up and I'll be wearing long sleeves for a while.

But I don't understand. I also don't understand why I would do this out of the blue so late in life? I am in counseling, but I am scared to bring this up with my therapist because I'm afraid that she will up my meds (which I hate) or want to do something else drastic. Really, I feel like this is an isolated incident and it freaked me out enough that I don't see myself doing it again.

I suppose I just wanted to hear from older people who have a similar story?