The main issue I see from your description is that "well..." is still fairly ambiguous, so while you mean "well, time to wrap up!" she sees it as "well, time for a new topic!". So it sounds like you need to be more explicit about signalling your intent to end the conversation.
I am terrible at ending phone conversations too, especially with my mom, who always has lots of random news she could keep sharing. What works best for me is making polite excuses. Even if the reasons feel a bit weak, it feels less rude than being blunt and less awkward than waiting for them, and it's a pretty common social signal (at least in my circles; I'm from US, and I picked this up in part from noticing how other family members ended phone conversations with me).
Usually these excuses take the form of household chores, since that's an understandable thing that everyone has to do. For instance,
- Well, I should fold my laundry before it gets too wrinkly...
- Gotta take care of those dishes before bedtime...
- I ought to start dinner soon...
Bonus points if you can mention these things earlier in the conversation! Usually there is a natural point, like: "so what'd you do today?" -> "Long day at work, and I have to get some stuff done around the apartment still! Good thing it's almost the weekend, right?"
Another option is to frame it as a favor to her, if she mentioned things that she has to do too. "Well, I don't want want to keep you, sounds like you have a busy day ahead!" is another very strong hint to end the conversation. If she is not good at taking hints, you could take this another step further and just say it straight out:
Well, I don't want to keep you, and I have to run some errands before it's too dark out. It was great talking!
As a last resort, you can also schedule your calls so that you have to go for some external reason, like calling before an event, or strategically starting some task that will need your full attention in an hour. I wouldn't recommend doing this consistently (as she may pick up on the pattern, and feel like you're avoiding her), but it has worked for me in the past when I felt obligated to call home but really didn't want to get stuck on a phone call.