Thank you all for your kind responses! 
Diamond, I often reflect on the similarities between bereavement and estrangement, experiencing both in quick succession definitely throws each into relief. When it comes to the time it takes to adjust though, I'm probably just being impatient for the hardest part to be done with, which obviously isn't how it works. If only!
Smileless, I haven't tried doing the things that have worked for the bereavement, I'm wary because the negative things wrapped up in the estrangement make it feel more complicated. I had a very positive relationship with the person that died, so missing them is so hard but at the very least it's straightforward. Because the estrangement is complicated by all the negative experiences, I worry that letters and keepsakes would end up being marred by that negativity. There's also a part of me that wants to keep them separate, to grieve them in ways that reflect the differences in the relationships... but that's where I get stuck for ideas. Things often feel "not quite right" with estrangement, in a way that makes it feel just different enough from bereavement to throw things for a loop.
Babs, I really like the idea of the balloon, thank you. It's not something I would do for the bereavement - I am still clinging on too tightly for that, I think! But for the estrangement, it seems like something that can honour the relationship (good bits and not-so-good bits alike), without the "risk" of getting bogged down in it. Even just imagining the balloon floating away feels quite peaceful. I wouldn't trust myself with a plant though unfortunately, it probably wouldn't survive me and then I'd be left with a different kind of visual reminder!
Cabbie, that idea is so simple but somehow so hard to remember in those tricky moments. I often forget that a well-timed cup of tea and a taste of something sweet can do a sturdy job if you'll let it! Thank you for the reminder
