Well done on putting yourself out there OP, and trying to initiate friendships. I hear you, I've found these things tough myself in the past and it can take up a lot of headspace.
Re this friend, I think it's too soon to pull the plug. As others have said, focus more on how it is in person than on the messages in between. Not everyone is that keen on or has the time for WhatsApp chit-chat. If over more time, she turns out to be flaky, I'd reassess.
I'd also try to broaden your efforts to make friends to some more people. I've found that when you overly focus on one person or one group, it comes across as a bit intense. Even if you don't think you are or are trying not to be, it comes across that way because it means so much to you and people hold back a bit with you, because it's just too much. If you spread your efforts a bit more, it dials down the intensity a bit and people respond better. Obviously, you don't want to spread yourself too thin, but I think if you do look a bit more widely and relax a little, you will find your people.
I'd also add that we socialise with people on different levels. Some people will be more occasional coffee friends, some people you go for regular walks or dinners etc with, some may become such strong friends that you holiday with them, for example, All of these friendships are valuable, but not everyone will become a really close friend.
I think when you've had friendship struggles (I include my old self here) it's common to take an all-or-nothing approach: someone is a close friend or not at all and if they're not going to be close or make steps towards that, then just forget it. Again it's that intensity thing. That person may be a very nice occasional friend, but they don't have the space for a new close friend right now. It's worth recognising generally that those lighter friendships can still be worth something, rather than diving in deep every time. Sometimes with a bit of time and space they MAY (or may not) evolve into something more. It takes time to make an old friend after all.
Keep going OP, it takes some practice.