But yes, I should be able to have this conversation
Just to make sure it was clear what I was saying, I don't think that it's a fault in you that you can't. It doesn't really matter whose 'fault' it is. But the dynamic is unhealthy, and if you can't talk to someone you're meant to be close to about your feelings, then it's your responsibility not to make yourself do it, but to be around them less, and find people you are comfortable opening up to.
For a long time I thought that 'having better boundaries' was about me learning to communicate my needs better, but it's actually as simple as finding out whether the other person respects your feelings, and if not, being somewhere where they're not! I spent ages in counselling, trying to work out what was wrong with me and what I needed to change about myself, and it turned out that there was nothing wrong with me, and all I needed to change was who I kept company with. All that counselling to find out that I didn't need counselling 
Think about how you feel with your closest friend, or that auntie you can talk to about anything, or... whoever you have in your life who does listen to you and respect you. Doesn't it make you feel like a different you? A better you? A grown up, self possessed, self respecting you?
If he's unresponsive when you've talked to him about how you feel, you need to find someone else or be on your own. It's not the behaviour of a loving partner.