My boyfriend is exactly the same hahahaha. It's not that he's bad at reassuring, my girlfriend is just a lot better to talk to when it comes to these things. Most of the time I'd prefer to turn to her more. The thing is I tend to go to my BFF first when I'm on the verge of an emotional breakdown, so I get my dose of support, and I fill my boyfriend in with what's going on in my life so he's not left out.

Okay so here's the stereotype (don't get mad people): generally, women are emotional creatures, we are so much better at letting emotions overwhelm us and other people because we're more compassionate and intuitive. Men have emotional 'buckets', it gets full, they need to tip it out and regenerate. Men can't ever comprehend that you need other people for support other than them because it hurts their ego, they like being the number one person you think of whenever trouble comes along, they like being the superhero.

Let him know there's nothing wrong with seeking support from different people for different situations. A partner can be number one in a lot of ways, but they can never replace everyone and everything, eg. your best girlfriend, your brother. I think the most important thing you have to let him know is that you're an individual, a lot of things you have to deal with yourself.