Why it's hard to forgive when you get older

So, other than the friend who is passing...

I'm dealing with several friends with significant issues.

One is going through a divorce, and that's messy.

Another was just involved in yet another accident; which was not her fault... except that the law of averages says she's got to be involved; these accidents don't just happen by themselves.  The law of averages says so...  and I have to be supportive because she's a very good friend and loyal as hell. 
But, that doesn't make it easier when she tells me that she is going to end it all, because nothing is going well.

But the kicker, is a friend/neighbor who grew up in an abusive household and has shitty tools for communication; and rather than address the real problem that she is upset about, will attack about something related but not relevant to the current outburst.  And, oh... does she rant.  (Yes, I know it's not a complete sentence.... get over it).
So earlier today, at the request of her roommate... I showed up to help said roommate with her new phone.    The gist of the complaint is that I didn't call and ask first, and that is intrusive.

I agree.  It is intrusive and I should be more mindful and not do that.  So, I will modify that.
However, the amount of unrelated diatribe heaped upon my head by this person while not addressing that issue (because that was almost not mentioned at all) was upsetting.

I got angry.  Because I am trying to be a better person, and I have learned that responding in kind does not work, I just said I'm leaving.
But, I didn't help her roommate with her phone, and I'm going to just let the situation cool off; otherwise I will say something I will regret that is hurtful whether it is true or not.
But, because I'm still angry and hurt....  I have to reign my mouth in, or I will say what I think.

I already know that is a bad idea.