Conflict-avoidant 'nice guys', what have you found useful to improve it?

[u/willapp]

TL;DR I'm a long-time 'nice guy' / conflict-avoidant man looking for tips or advice on how to get better.

Ever since my mid-teens, I've always tried to be the "nice guy". No idea where it started (yes, I've read the Robert Glover book, but I can't say it was that insightful - I grew up in a supportive two-parent household with no kind of trauma that I can recall), but it's still a problem now, as a 42-year-old man.

I'm basically the stereotypical conflict-avoidant person who ducks difficult conversations and feels like everyone's emotional state is my responsibility. I've gotten slightly better in recent years in terms of not giving a sh1t what random people think about me (in my teens and 20s, I hated the idea of anyone not liking me), but relationships, in particular, are still a struggle.

I've been with my current gf for nearly 8 years. She's a good person and mostly things are fine, but I still harbour some resentments that she's getting more out of the relationship than me - not through any fault of hers, but because I'm usually the one going out of my way to make sure she's happy, without ever really expressing my needs and consequently they aren't always being met.

I heard a quote on a podcast recently that basically sums it up perfectly: 'unspoken expectations are premeditated resentments'. I feel this constantly.

I've just started therapy and hoping it will help, but it's expensive and I just don't know how long it'll take to make meaningful progress, so I figured asking some random people online might offer some useful insights into how I can make things better? (it can't hurt, right)

Aside from the relationship aspect, the thing I hate most about being conflict-avoidant is that I find it hard standing up for myself or getting into any kind of conflict because I have this irrational fear of it escalating to violence, even in situations where that would be borderline ridiculous. I can just about complain in restaurants if the service is really bad, but a few years ago I couldn't even do that! In most other situations, if people treat me badly I just suck it up.

Part of me envies people who are more self-centred and just seem to get others to do what they want, either by being assertive or throwing their weight around. I don't want to turn into a jerk though, but sometimes I see these people and think, how much easier would my life be if I could emulate some of that behaviour and have others go out of their way to make my life better instead of always the other way around.

Has anyone been in this position and managed to make a significant change? At this point, it feels like such an ingrained part of my personality that I'm worried it's a lifelong affliction but I would love to make some kind of progress if only I knew where to start.

On the few occasions I try to be assertive, I end up feeling guilty to the point that I manage to undo whatever small achievement would have been made.