(Not ignoring the first half of your post- I'm sorry that's happening and pray it will get better/get worked out for you, your dad, and the office manager woman. That all sounds like a lot of stress for a lot of people and I hope it gets resolved sooner rather than later.)
As to what you described as "generational sin"-
This, I can empathize with. It's very similar to what my husband is going through; he is very prone to angry outbursts and being easily frustrated at little things (I specifically recall a time he was trying to fix a space heater that kept making this high-pitched squealing noise, and he got really upset, hit the heater hard enough to break it, and walked out...in front of me and both kids). His dad is the same way, only worse- I've seen my father-in-law get so angry over the silliest things when he could have simply asked for help or took a minute to step away and think about it. I can't say whether his father was this way or not; he was a police officer and was killed during a seemingly routine traffic-stop when my father-in-law was a baby...but I imagine the temper has been passed down through at least a few generations. I already am dealing with it in both of my kids.
So here's what I want to say to you on that-
The fact that you recognize this in yourself, that you already realize how bad of a problem it can be, and that you are already trying to find ways to curb it, or even stop it from happening- that's huge. That's a highly commendable thing, and I know how difficult it can be from watching/helping my husband to get his temper under better control (and it certainly can be done, he's made amazing progress, and it sounds like you have as well, even if it's starting to crop up again).
I would say it's probably not the best idea to simply avoid ever feeling angry about anything- that sounds a lot like bottling it up, which might be why you're feeling the anger coming out again recently. It's not wrong to get angry about things, and it's not even wrong to feel really frustrated at something as silly as building a bookshelf- it's how you release that anger that matters. Yeah, blowing up and putting a hole in the wall...not great. It would certainly be a scary thing for a wife and kids to see (it IS scary to see that kind of outburst). But you can choose to walk away...to go outside...kick an old tire...cool down.
Everyone gets angry or frustrated sometimes. It's okay. And your future wife will probably understand that. But it's important to find a way to deal with those feelings that doesn't scare people, and doesn't require a repair job on your wall. I know you know that already; I hope you keep it in mind though, when you start getting down about letting your temper flare up.
Thank you for saying all of that. It's really easy to complain about singleness and whatnot, but I have to say that it's by God's grace that I made a mistake and put a hole in the wall now rather than down the line when I do have a family in the house too. You're totally right that bottling it up isn't good. I don't know, it was weird before. I guess I just decided I wasn't going to let things bother me, so it wasn't so much that I was feeling anger and suppressing it, but I just wasn't getting angry in the first place. It was crazy how I was able to flip a switch that way, but I felt awful about making my mom cry, and the first thing she did when she started crying was walk up and hug me. I guess that was powerful enough to cause a lasting change in me. I'm still nowhere near as bad as I was then in terms of being easily triggered. The hole-in-the-wall incident isn't normal for me either, but it was something that absolutely should have never happened in the first place. It was like as soon as I realized what I did, the thing I was mad about didn't matter anymore.
But you're so right, it's just about dealing with those feelings the right way. Taking five or ten minutes to sit outside and breathe and pray. And one thing that has helped me a lot in the past is practicing gratitude as well. It made a difference when instead of walking into my house thinking about how much the day sucked, I made myself think about the fact that I'm walking into a house...that I own...that is warm and contains food and clothing, plus other comforts that I don't need.