1. Not asking for help. Studies have asking people for help and advice actually makes you seem SMARTER, not dumber. So don't be afraid to ask. You don't need to figure everything out on your own.
2. Trying to make bad relationships work. Putting your time in on a good relationship is worth it. But we spend huge chunks of our lives trying to fix bad ones. Which ends up wasting both people's time. So sometimes a clean break is good.
3. Dwelling on your mistakes and shortcomings. Learning from them is great. But dwelling on them just wastes time and messes with your self-esteem. And it can hold you back by making you too afraid to fail.
4. Worrying too much about other people. Whether it's what they think of you, or just being jealous of stuff they have that you want. We tend to focus a lot of our energy on other people instead of ourselves.
You can also be TOO supportive. Being there for friends to lean on is great. Just be careful about sacrificing your own happiness for theirs.