That sounds to me like she doesn’t even know you are infatuated with her, correct? You mention that you never told her you’re in love. What you are doing is unfair both to yourself and to her: If she thinks you are friends, she has no reason to treat you any differently than her other friends. So it’s understandable she might take some time to answer if she’s busy. How would she know you are expecting more of her?
It is unfair to yourself, because you’re keeping yourself in that limbo by obsessing over it. You are expecting her to magically behave in exactly the way you want - without ever having communicated your feelings and expectations. And then you get stressed because another person didn’t behave the way you wanted them to. That’s like getting depressed because the sky is blue and you got it into your head that it ought to be yellow. The only reasonable thing you can do is work on your own expectations and reactions to other people. You have no influence over other people’s feelings, how they behave or whether they like you or not. If you allow things you have no influence over to take control of you so much, you will never be happy in life.
Even if she told you today “I love you” and you get together - how would a relationship work if you can’t even communicate a simple need such as “I noticed you take a long time to answer - is something up? I’d appreciate if you could at least write a short ‘I’m busy, let’s text later’.”
And yes, the people who tell you to work on yourself DO get it. One person not reciprocating your feelings should not throw you into such a deep depression and if it does, you need to work on yourself and not date.
“Just do it” is helpful in some cases, but mostly not. E.g. you think that a hobby is cool but you don’t feel like you could start it? Just do it, take a course, try it out. It becomes unhelpful quickly when the realities of your life are just different. Telling in unemployed person with debt who is fascinated with flying to “just get a pilot license” ignores their reality. But telling a business analyst who’s interested in manga but feels like this hobby would destroy his image, to “just do it and buy some mangas” is totally valid.
I have been struggling financially for most of my life and have received way too often the unhelpful advice to “just do it. Live a little.” Just book that 100€ flight to Italy and see Rome. Just get a smartphone, everyone has one now! (That was when smartphoneplans were very expensive here and I couldn’t justify such a high monthly cost. Yes I’m older.)
There is way too much “just do it” advise by people that live in their nice little bubble of a well-off, supportive family system and never realize that the only reason they can “just do it” is because they never had to eat rice with tomato sauce for 3 days in a row because there were only 10€ on the bank account by the 26th.
On a similar note, “just get a job, just learn something more profitable/in an industry with high wages” is also an often unhelpful advice. Not everyone can be good at everything. And not everyone can just uproot their lives and go back to school for a few years. Yes, some people can do amazing things like get a masters degree while working full-time and having kids. But this advise, too, ignores the reality of many people. If you have no support system or if you simply aren’t cut out for the currently profitable jobs, you can’t just magically switch careers. And even if you do: things change so quickly and there is no guarantee, that the currently well-paid job will still be like that in 5-10 years.