When to end a relationship?

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How do you know when to end a relationship and do you have any personal experience to share? I met a woman four years ago and we fell in love almost immediately and a year later she became pregnant. We had a beautiful little boy who recently turned two and he means the world to me.

Unfortunately it feels like our relationship has gone down the shitter for quite some time now and I feel that I deserve better but can't stand the thought of not seeing my son every single day. She never cares about my opinions or what I feel in various matters, she's almost always in a bad mood and it really brings me down as well. She's been very negative throughtout our whole relationship but it has gradually become worse. In the beginning I always tried to joke around and make her happy but eventually you get tired of doing so when there is no result.

We never spend any time together either and the few times we've had the ability to do so she ended up choosing to do other things instead of spending time with me. I know this sounds like a bunch of moaning but I seriously don't think this relationship will get any better in the long run and feel that it's best for all of us to end it sooner rather than later but them feels when I think about being away from my son... :(

tl;dr 4yr relationship with a 2yr old son. ANy advice on if/how to break up?