Being vulnerable can seem daunting, but that doesn't mean its aftereffects have to be. Vulnerability is a gift that keeps on giving when we learn how to tap into it. Still, humans tend to have an aversion to opening up. Off-hand, there are two major reasons I think we find it difficult - the want to be invincible and the fear of being judged or misunderstood.
If I say that I don't know what it is about seeming invincible that seems so appealing, I'd be lying. Superheroes, action stars, and larger-than-life characters with chips on their shoulders are all built on this very principle of invincibility. However, if we're using them as an example it must be mentioned that they, too, have their confidantes.
In what can be a quest for invincibility unknown even to our own selves, we may try to prove that we don't need anyone and most certainly don't need help. Of course, personal drive, motivation, and vision have their own place, but that doesn't mean you won't need help along the way - it's not one or the other. Check-in with yourself and see if the need to always be "on" creates a mental pressure on you to always be feeling a certain way or always be in a good mood.
As for the fear of being judged or misunderstood, that can be a very real fear if you aren't sure who to go to. If you're in that position, try finding a therapeutic 'what' to go to; journaling, painting, creative writing, building, reading, crafts, home improvement, music, organization. Anything that helps you clear your mind, feel more like yourself, or feel more present with yourself will do you a whole world of good.
And if you are sharing how you feel with a person, know that holding it in can be riskier than letting it out. Think about it this way: you know exactly whom you're sharing your thoughts with and you go to them with a level of trust that hasn't come out of thin air. If you hold it in, there's no place for it to go and it might grows. Also consider that the other person will feel relieved and grateful for you sharing your feelings - it's a chance you trust yourself to take, so please, please trust. Take that leap. If it doesn't end up the way you hoped it would, communicate that respectfully if you are able to. If you aren't, you still have to be proud of yourself for taking the jump because in doing so because you showed yourself that you care for and respect yourself. That sets a precedent unlike any other, one that you will continue to follow for years to come.
Yes, consider falling if you take the leap; you will still be able to get back up. But also consider that if you leap, you may very well rise.