Self advocacy

I consider myself a fairly empathetic person. I also have been considered by others selfish/self absorbed, and they have a point at times. So, I find myself having serious difficulties kindly and effectively advocating for my own wants and needs when something has happened/a situation arises in which my wants/needs clash with another person's needs. Or their wants about how they meet their needs. Don't get me wrong... Someone actually fucks with me and, if it is safe to do so (and probably sometimes when it's not), my mouth opens before I can even think about it and I'm asserting myself. But when I can clearly empathize with the needs of the other person and recognize that mine are still important but possibly not as great and can be met in other ways, even at inconvenience to me, I clam up. I think in part because it's difficult for me to be assertive about my needs without either very very strongly and powerfully stating them and not leaving room for anyone but sometime equally as assertive to respond with a team approach as opposed to an oppositional approach, or I'll overly verbalize the empathy in a way that really just makes it uncomfortable for everyone. Help? Please?