Being able to trust yourself above anyone else isn't just important — it's crucial. You have to be able to hear your voice before you hear (and listen to) anybody else's, and that's not something that comes naturally to a lot of people. This is to say: you must be your own sounding board before you can really listen to anybody else's opinion.
But then there are times in our lives when we aren't thinking straight — we're too overwhelmed by our circumstances to really listen to logic or reason, and often we're too inundated in emotion to even know that we aren't in the best frame of mind. It's moments like these (which happen to us all) in which the word or advice of a well-intentioned friend or family member can be life-changing, if not life-saving, and I'm sure you can all recall either a moment in which you tried to intervene on a friend's crisis or when you were truly going through something and didn't realize a friend was trying to help you until you were thinking clearly again.
Knowing who to listen to before we get lost in the heat of a situation is a really important skill to have, it's a matter of clearly thinking through who your tribe is, what messages are worth listening to, and how, even in the middle of whatever life is throwing at you, you can learn to trust someone when you aren't thinking straight yourself.
You're Dealing With A Substance Abuse Issue
If your problem surrounds or coincides with a use or over-use of a substance, that's when you most need to listen to someone whose mind and body isn't completely bogged down by a manic desire to sustain the high.
Who you should listen to: Someone who has been through a substance abuse issue before, be it a sponsor or trustworthy (and recovered) friend. Someone who hasn't been through this particular kind of hell won't understand, and someone who has will know what it takes to get through it.
You Seem To Be Stuck In The Same Destructive Pattern
If you feel like you're somehow stuck in the same patterns that lead to your seemingly inevitable unhappiness, you should ask someone close to you to point out a habit or desire or trait that you may be blind to (that may very well crack the code as to why you keep reverting back to your old ways).
Who you should listen to: Someone who knows you very, very well and has your best interests at heart (isn't in "competition" with you in some way) and doesn't "need" anything from you.
Numerous (Well Intentioned) People In Your Life Are Saying (Or Doing) The Same Thing
If everyone in your life is pointing out the same problem, it could certainly be a coincidental lapse in their perception, but it is more likely something you haven't noticed about yourself yet. If everyone seems to lose touch with you after a while, or says you do [one particularly infuriating thing] it may be worth exploring, and seeing whether or not it's truly an aspect of your personality that you can't change or if it's a result of an internal issue that's causing you distress elsewhere.
Who you should listen to: Someone who has commented on the issue before, who will be compassionate and kind and objective as you move through your understanding of it.
You're Too Emotional Over Someone To See Them Objectively
This is the big (and often most important) one. It's always in destructive relationships that we become most convinced we just have to get through the "normal turmoil" of love. It's always only after-the-fact that we realize our friends and loved ones had a point in being instinctively turned off by them, or just not generally vibing with them as people. The people in your life will most often be inclined to try and bond with your new S.O., and to be hyper-conscious of who they are (in case you have any hormonally-induced blind spots). This makes them the people to listen to, even if you don't always want to hear what they have to say.
Who you should listen to: Someone who loves you and who isn't attached to the outcome of your romantic relationship (i.e. not your mother who wants you to get married soon, but a BFF who only wants the best for you).