Hi Angst,
I've been there too - and still am in some ways although not all the time. For a period in my life (while i was asleep) I had several people in my life who expected too much from me, and surprise surprise these people were never there for me when I needed them. But I didn't see that because I was focused on their expectations and needs.
I did lose some people by becoming more assertive, but it was worth it because honestly we are all busy and can we afford to spend so much energy on people who resent us for having needs? That is unrealistic for them to do that, but they are out there, believe me I know.
I am now practicing assertiveness and finding that I do still feel anxious when I have to be assertive but the more I do it the better I become at stating what I need or expect without feeling bad. You will get better at it too. If I see the potential for a dispute with someone, I think about it more now before going into it. Maybe I don't have the energy so I avoid it for a day or two or more. I take care of myself for a day and go from there.
I don't know about you but, taking care of myself (focusing on my own needs) instead of focusing on another person including trying to resolve a dispute with them, is hard!
I still have some people in my life I end up in conflict with and who challenge me and force me to be more assertive than I like to be. But I space out my interactions iwth them. I find that when I step back to look at how they are behaving, I notice that they are also demanding that the dispute be resolved 'immediately'. The fact that I am off taking care of my own needs for a day or so doesn't sit well with them. But that doesn't matter to me anymore.
It helps that I found a couple people (finally) who say "hooray" when I am taking care of myself rather than "stop it and pay attention to me" which is what I had for so long.
I'm rooting for you! keep posting and keep at it. Becoming assertive does get easier.