Every person is going to have a Mercury placement. Mercury as a planet is going to indicate how an individual communicates, expresses intellect and generally deals with knowledge and perception. How Mercury is expressed in a chart is going to depend on it's sign placement and aspects. There are potential strengths and weaknesses to every Mercury placement.
There's a couple of things in your chart that might contribute to feelings of inadequacy around intellectual ability. Some Mercury related and some not.
You have Mercury in Leo in the First House. That in and of itself is a placement that implies that there’s a strong relationship between your sense of self, your sense of pride and your intellectual ability. Being able to present ideas in a way that’s unique, engaging and compelling is fundamental to who you are. The question of how to understand your intellect in the grand scheme of things is inextricable from the question of where you fit into the world, as well.
For the most part, you seem to have a fairly well placed Mercury. There’s two aspects and a semi-related thing that might complicate that.
First, you have Mercury in hard aspect to Neptune. That’s an aspect that creates both mental restlessness and a sort of ephemeral quality to thoughts and ideas that can make it hard to actualize them. Here’s two descriptions of the placement:
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This dynamic couples with that of your Mercury-Chiron contact, which indicates that you are much better at encouraging the intellect of others than you are at necessarily expressing your own. You have some conception of intellectual brilliance--you know what it should look like and even how to guide others to it--but you always feel a bit awkward in attempting to express your own.
That dovetails fairly well with the air sign part of this equation. What [MENTION=10082]Starry[/MENTION] said was pretty on the money with this.
You have a really packed 1st house. In astrology, the 1st house is the house of self, and is opposite the 7th house which governs relationships. We tend to project the energies of the planets in our 7th house onto those around us, especially our romantic partners.
Because you have so much stuff going on in your 1st house, it creates an imbalance with your 7th house, which is empty except for your true node (we’ll get to that in a moment). That makes your 7th house a very sensitive point in your chart. Your 7th house is ruled by Aquarius—an air sign—that is associated with vision, genius, and “higher†knowing/ideals. (As an aside, again, [MENTION=10082]Starry[/MENTION] intuitively nailed it on the head in her comment about the line between “genius and madnessâ€â€”mad scientist and "eureka" moments are both very Aquarian concepts.)
Either way, being confronted with those sorts of archetypes creates an instinctive feeling of “lack†in you. Here is the person you most want to be, but fear you will never become. You maybe find yourself surrounded by people you recognize as brilliant, and always feel a bit in their shadow.
Now, the real gag of the 7th house is in the fine print. As with all projections, even though we tend to externalize what’s in the 7th house, it’s really ours all along. In doing the things that come naturally to you intellectually—i.e., pursuing knowledge in a joyful and generous way—you kind of end up being that eccentric genius guy anyways.
That brings me to the last point regarding your true node that is also housed in your Aquarius ruled 7th house. True node is about life’s purpose and the kinds of lessons that will make our lives most meaningful. You're always going to have the most work to do when it comes to “owning†your intellectual identity. It’s always going to push you in the places that are the most uncomfortable, but will also compel the most significant personal growth. When you dismiss yourself and don’t embrace the revolutionary quality of your own mind, you play yourself, and kind of short change the rest of us as well.