The outside view is usually more accurate than the inside view
How much easier is it for you to solve your friend's relationship problems than your own? Obviously your friend needs to break up with her boyfriend, or just bring up the obvious problem in her relationship, but no, your own relationships are more complex than that.
Most of our biases come from being a subjective self. We have internal experience and are caught up in our own stories so that we think we are the exception to every rule. No one personally feels biased, even if they intellectually know that they are.
Because we can get so caught up in the small details of problems and questions when they are personal, we are likely to undervalue and overlook certain things, while putting far too much weight on more emotional aspects of it. But if we can take an outside view, one where we disregard all of the specific circumstances that seem to make this case special, and just look at the big picture, we will usually also have a clearer picture.
(It's the difference between the advice you'd give to a friend and the advice you follow from yourself. One way to take this view is by asking yourself: "Would I be genuinely surprised if this didn't go how I'm expecting it to?")
When planning, it's more effective to ask yourself how long on average a 5 page essay takes you to write, rather than how long will this specific 5 page essay take you to write. It's calming and reassuring to get relationship advice from your friend as they often have a clearer view of your situation because they have less knowledge. They can easily see the bigger picture. There is a clear path, it's just hard for you to see it so close up.
TLDR:
When we try to solve personal or emotional problems we can easily get caught up on small details that distort our views and decisions. Taking an outside view where you purposefully ignore the specifics of the situation can often be more accurate and helpful.
References:
Farnam Street? Thinking Fast and Slow?