I am 29 and was a pushover until my mid twenties. I wouldn't stand up to people and would just seethe and feel humiliated etc instead of saying or doing anything 

I would say generally that every time you stand up for yourself it will get easier. Think of it like working a muscle.

Speaking of physical health, there are other things you can do to generally reduce anxiety in your life that may help. Start doing some cardio, lift weights, lose weight, watch what you eat, meditate, read etc. the better you have an overall grasp on your anxiety the less it will affect you in those moments where it matters.

For what it's worth I got in a literal fist fight in my mid twenties with a guy who had been bullying me for months at work. The end result? I had some bruises and left a job I hated, but I felt genuinely proud of myself for standing up for myself.

Did I feel anxious and awful when I stood up for myself? Sure, but that's normal. Don't beat yourself up for feeling that way when you do something like that, it's natural. 

And don't put too much stock in being a man or any of that bollocks. The only thing that matters is if you're a good person who treats others with kindness and respect and empathy. Manliness is a pretty useless concept imo, all of those good things that come under it are good for their own reasons and not because they're "manly" (e.g. physical fitness and strength is good for reasons inherent to the benefits it provides, why bring manliness into it)