1) Not setting goals. When I was about 20 (ten years ago), I was making a shit load of money from a niche ($300k per year) and I got lazy (depression maybe too) and pretty much walked away from it, but it was on autopilot so continued to make six figures for a few more years, but kept declining. Looking back,t he only thing I could think that could have changed things was setting goals and sticking to them. But at 20, it was hard to not be happy with what I've done. But the truth is if I could have even put in a few hours a day, I would have been making 7 figures. Now with competition today, things are 100x harder. To give you an idea, I created a site in about 2 days, it was a list of pharmacies for seniors and other people to get prescription drugs cheap. I pretty much just copied the list from a free site. All I did was submit it to Yahoo directory (think it was $200 at the time), and two days later that site was making about $100 a day, and I never did anything with it again.
2) Giving up. This ties into my above story. Bigger competition came into the niche, and I got discouraged. One was a big time company with a lot of money, so basically made my site look like an amateur. The other big competitor spent a ton of money on blackhat (which I wasn't into yet). These two guys destroyed the rankings, and looking up at them, trying to get myself back into it, I felt I was at the bottom of a mountain that was covered in ice.
But finally, I did get back into it, and realized, I may not have the money to do what these guys did, but I'm smarter than them to capitalize on other areas of the niche (smaller keywords). Then I also became a regular here and realized things were so much easier lol, and I did have a chance. So I battled back, and I continue to climb and slowly take chunks out of their empire. How's this for white hat. After about ten years of my site being online, it had about 150 natural links, was depressing. I was about #60 in google for the main keyword which is 150k+ exacts per month. Frustrated that there was so much crap above mine, I started doing some BH. Today, I am #4 in Google, of course behind the two competitors, and wiki. But I also have about 40 other sites most ranking in top #5 for their keywords, and I'm developing another 60 sites.
I have a son now, and he is my motivation. That and not wanting to ever be 'normal', and live a 'normal' life.
Sorry, long rant.