She's probably noticing you seeing the pretty girl because she senses all the rest of it, and that is too big and overwhelming, so grabbing onto a small easy thing, like if you look at a pretty girl, is simpler. (And simpler for you, too. You can get cross if she gives you the hawk eye if you look at a pretty girl, and not have to get real about the much bigger problem of your depression and anxiety.) Survival issues arise if one has to talk to one's spouse about big anxieties, when if the fight is only over something not very serious, you blow off a little steam and don't have to address the scary stuff.
See a counselor if you can. You've got five young kids who need you and your wife to be a couple.
The other point is, if you see a counselor, do it for depression and anxiety. You are blaming your life for making you feel trapped, but in truth, the depression and anxiety are what is making you feel that way, and if you were in a different life situation you would still have the depression and anxiety. Not saying you should not make some moves to get more out of life. But one move should be to a counselor, and right away. It's not your wife or kids' fault that you are depressed/anxious.
To be honest, I'm not sure saying you feel trapped after producing 5 kids with your wife is fair. :>) You kind of made your bed. I"m a fan of couples once they have a family staying together unless there is damage going on for the kids. We make choices when we have kids.
Depression and anxiety (as anniebrooke says) is no one's fault and is often an organic issue. It's treatable. That's the route you should take before thinking of breaking up your family.
Hawk eyeing a pretty girl is bad manners. It's not cheating or that big of a deal but just impolite. I'm married too and my husband is a man as well. I don't expect him to never look or admire other women but really, staring at her in front of me is rude. Just plain old bad manners. He's smart though and if he is looking and he sees me looking at him looking, he quickly says "you would look good in that dress.". Then I smile and we go on about our day. But if he is just lewdly looking, ya. It's just not polite to your significant other. It's not bad. It's just not a feel good moment for your partner. So, make an effort to either soften it or make a funny comment.
So, I kind of gave you my brutal advice here. There is such a thing as 'mid life' crisis and maybe you are having it early since you got married early. Don't know. But I'd work on your depression and anxiety first before you do anything drastic. Your kids are worth that effort.
I am also wondering if she feels trapped. My little sister (whose kids just went to college) told me recently how "stuck" she felt when she was younger. The only way out was through, as the saying goes. She had to be there and be the mommy and make the lunches and arrange the playdates and basically sweat it with a lot of gruntwork until they were done with high school and out of the house. Now she feels, by contrast, a lot more free. Luckily she and her husband didn't wind up taking this feeling out on each other, but she is so glad that time is done, love her kids though she does.
I am thinking that if you are focused on your wife as the problem, you are not actually going to solve it by (for example) building up enough of a sense of grievance against her in your mind that you can justify leaving. I think you will be just as stuck or trapped if you go as if you stay. This is because, well, five children. They need you and believe me, if you think dealing with an angry wife is hard, try doing all he handsprings and timing and calling and coordinating required when you have shared custody. My divorce-lawyer friend said to me recently, "NEVER get a divorce." In short, anything else is easier. (I assume you aren't being so unrealistic as to think you can just wash your hands of the whole family and walk away.)
If you and your wife can't rekindle some affection for each other, can you at least have a talk (moderated by a counselor) about being civil to each other? It sounds like she is being critical and you are being self-pitying and blamey. You could probably get to a point where you are at least both civil. But it might take some counseling help.
A) I think you really can't leave at this point. It's not a simple matter to leave. There is the hassle of joint custody, there is the sadness of your children, and there is the money. Also, you don't sound like the kind of man to leave helpless kids in a bad situation, and it sounds like it would be bad if you were not there. When my parents divorced, I felt like my dad was saying I should not have been born. Do you want your kids to grow up thinking this because of your actions?
B) You also sound pretty traditional. I'm married to a traditional man, and I like it. But the one way it is really hard is that he doesn't see how difficult it is that he assumes all he has to do is "work to support the family" (i.e., earn money). He comes home and sits down to have a beer or watch TV with an air of someone who has no more responsibilities except to demand to know where dinner is. Tell you the truth, I've worked at a paid job and I've been a mom home with a child, and it is a ton easier to work at a paid job. It is so much more satisfying, respectful, finite, and so much simpler to just be able to come home at the end of the day and relax. With kids, there is no beginning or ending point and no milestones and no respect and no time off. Your wife has this times five, and it sounds like it is taking her down. I will not suggest what you should do about it, this is your life and hers, but I do suggest you drum up a ghost of sympathy for her, even if in your mind this marriage is not going to continue past your last child going to college. She's got a very tough job and yours is easier than hers. I don't suppose you will believe this but it's true.
Good luck to you. Stick with the counseling, I think it will help even though you pooh-poohed it above. Put your kids' needs first, remembering what will happen to them in terms of lack of stability if you dump on them. If you ever want to, some time when you and your wife are both calm, propose to her a truce, so there is less yelling in the house, and you could even say you know it's over between you but you would like to live in a civil fashion until the youngest is out of the house. It might sit her up on her hind legs a little, and get her to actually consider counseling.
Good luck to you.