This can help you do what you want more often. Use it together with everything else. Small wins are important because they all reinforce each other.
The trick is: since there are so many different things you can do, in some situations you can set a constraint along the lines of "I will only do the things I like/want". Let's call it "the selfishness constraint".
The important bit is that you will still have so many options left that it won't matter. If this holds in a given situation, then selfishness is fine in this situation. That's the important bit.
For example, if you are having a conversation, there are so many things you could be talking about, that it feels like a complete injustice if you have to talk about something that's not interesting to you.
Or—if it doesn't feel this way to you, but you can kindle this feeling in yourself. "C'mon, I bet there's something we can talk about that is going to be interesting for both of us." Or in some cases you might even go for "C'mon, I have agreed to this, I'm spending my time on you, so at least it should be somehow good for me, right?".
It took me about a year to start routinely thinking "Hm, there are so many possible things to do, I can limit myself to only the things I like and it'll still be fine".
It has legible benefits—e.g. other people appreciate it when you're enthusiastic and in a good mood, basically—but I don't know if you can get there by thinking about the legible benefits.
Repeating "I have dignity, I have dignity, I have dignity" over and over might work out better. "I will only pick up the projects I actually want to do", "I will not eat anything I don't quite like if there are so many things I really like", etc.
If you manage to do each of those 10% more often, they all start reinforcing each other and you will get momentum going. This is why it's worth doing even if it only works 10% of the time. It will start snowballing at some point.
So, here are two concrete suggestions.
First: The simplest thing to start with, for me, was "I will go pee immediately when I want to pee, not waiting for a break in the convo, not waiting till I finish the tweet, etc". Wanting to pee a) happens by itself and b) you can't ignore it, so it serves as a reliable reminder of the general principle. And the benefits that you can use in the internal argument are numerous—"I can't focus on the task and be useful when I have to pee", "I can't focus on the conversation and really listen when I have to pee", etc.
Second: In conversations, always do only the things you feel good about and want to do, nothing else. Never try to do something just for the sake of helping the other person. And if this condition filters out all possible things you can say—don't say anything.