Yeah I feel this way all the time. I'm terrible at standing up for myself and sometimes I end up apologizing when something was clearly not my fault.

It's sometimes useful to figure out where it comes from though. I know that when I was younger standing up for myself meant retaliation and being shamed by parents or others, so I quickly learned that it was "better" to just succumb to it and feel guilty about being assertive.

It's a work in progress. I suppose it's just little steps, knowing that your opinion matters and that your worth isn't inherently dictated by whether it coincides with other people's or not.

I don't have too much advice, I'm still working on it myself. But good luck.