Feel bad about the way a friendship ended.

[BonnieBumble]

Years ago I had a best friend, my friend was very intense and had issues that neither of us acknowledged. I was going through quite a difficult time which my friend was not aware of, I was not a good communicator and rather than address the issues with my friend I let her down knowing that my actions would lead to
the end of our relationship.

At the time I didn't think I was being that
unreasonable, I now realise that I was in the wrong. This all happened 20 years ago I feel guilty and sad that my inability to deal with confrontation and poor communication skills led to the end of our friendship. I have seen the friend once or twice since in passing and we have been civil but not talked.

What would you do? Should I just leave it and accept that it was a long time ago and we were both very young or should I contact her and try and explain? The latter will not be easy to do.

I would write her a letter. I had a long and close friendship with someone which ended abruptly over and arguement that had probably been a long time coming. She wrote me a letter two years later and we got back in touch. We are friends again and although we will never be as close as we were (probably a good thing), it is an amazing relief to no longer feel guilty about it and to have her back in my life.
A letter will give you time to craft what you want to say and be easier for you I would imagine plus it will give her time to reflect before responding.
I hope it goes well for you.

I do agree with Casmama the only thing I would add to that though is-

Who is this for? (I am extremely dense and don't think I've gleaned all the context from your op!)

If you have been holding on to these feelings because you regret what you did and want the chance to make yourself feel better but the apology and explanation may bring up things that would be hurtful and/or unresolvable for her then I wouldn't do it. It's from a time past in both your lives. I would write a letter and burn it and try and forgive myself by accepting that I have learned from what I did, regret it and would never behave like that again.

If your apology might help her dump an old hurt/be beneficial/at worst be benign then I would go for it and just make sure she knew the aim was to apologise and make emotional reparations to her and it was all very much in her court. If nothing comes of it at least you can leave it behind knowing you did the best to sort it out. Hopefully it will make you feel a lot better.

But - I think you need to cut yourself a little bit of slack regardless, 20 years of this building up I think is an ample price to pay.

I wouldn't be doing it to ease my guilt, I'm not sure why I want to do it really. I think so she knows that it wasn't her it was me. I'm not sure. She was a strong character but was also often bullied and I feel bad that I was yet another person who wasn't there for her.

As a teenager I had terrible communication skills and if there was a problem I would run away rather than confront it, it has taken me many years to change this pattern. I'm also more self analytical now, I can see how it must have looked from her pov when I let her down. At the time I really didn't think I was being unreasonable and my mum said that I was doing the right thing. However looking back I can't see how I was anything but unreasonable. I think in my family we don't really do communication or analyse anything and I think mum thought I was in the right because she would have handled it that way too.