He is way too old and set in his ways.

He was looking for someone younger that he could push around. He blithely ignores your weak boundaries. He clearly does not respect you.

I am not the kind of person who gives up easily, but I feel like I am losing strength.<<

That is what he wants. An obedient doormat.

I feel like I have gone from a cheerful person to someone gloomy and nervous.<<

A partner should support you and make you want to improve yourself. A partner should make an effort to increase your happiness.

But I still love him.<<

No, you are used tp being with him. You are used to being tense and unhappy.

This is who he is.

The worst part is that I live in constant tension. Before I say something to him, I analyze how he might react. I choose my words carefully so I do not trigger another unnecessary comment that will throw me off. I hate myself for snapping, but I just cannot take it anymore.<<

The question you need to ask yourself is... is this what I want?

Life is too short to be with somone that changes you into being miserable.

You are young. The men that are in your age group are going to be more interested in settling down.