It isn't that you can't/shouldn't share your feelings, but it's about the overwhelming feelings you're causing in your wife.

It's not very 50/50 if you can vent to your wife but she can't express her upset about how you are making her feel. It's sort of how things go - one thing leads to the next. It doesn't make what you feel invalid, or less important. And no relationship will ever be exactly 50/50 - there will always be times where it's predominately focused on one partner and then the other.

Is complaining at your wife the only way that you vent? Do you do anything more constructive, or confide in others? If you are only dumping everything on your wife then that isn't very fair to her, because you'd be spending all your time sharing negatives and not positives with each other.

And the problem here with your sweeping generalizations is the fact that it's been wearing at your wife for a long time. It isn't like it's the first time you're saying something really negative - when you first started feeling like this all the time she was (probably) quite supportive, or at least more supportive than she is now. But it's a pattern, and you are probably making your crappy life into a crappy life for her too.

Like...

I'm single, so I am going to use friends and exes as examples. When I'm stuck in a depression or am dealing with way too much stress, I never, ever, rely on just one person. I know that dumping everything on one person is going to overwhelm them (and has!) and that when it happens they aren't really ever getting any of the good-me. It always starts out with them being really supportive because that is what friends are for. No one (at least not anyone that I considered to actually be a good friend) has ever gotten upset with me or made the conversation all about them. But if it becomes consistent and essentially the standard tone of the friendship, eventually they're going to start feel like I'm using them and get overwhelmed at their inability to help and will leave.

It's all about balance really, and right now you and your wife don't have much balance at all.

ETA: (hit send too soon).

I've also had the opposite, where someone used me as their dumping ground allllll the time. I wanted to help, I really did. I cared about them. But eventually things became a pattern and I didn't really exist in their life for anything but as someone to vent to and get help/sympathy from. I wasn't in the friendship/relationship at all - I was just the receptacle. For my own health I eventually have to leave those friendships because they become toxic - if I expressed myself at all I would be guilted or blamed or something. It wasn't fun. It would get to the point where I didn't feel like I could share anything happy in my life - or anything negative. Everything had to be about them and how miserable they were. They really aren't fun relationships/friendships to be in. You watch someone you care about digging themselves into a hole, and it gets to the point where you have to either leave or give up having any equality.