(Sigh) No one ever learns the lesson, or drills it into their head... so, I'll say it again:
1. Texting KILLS relationships
2. This is especially true in the beginning
3. When you're not in front of her, you shouldn't be in contact with her until you're ready to ask her out on another one-on-one in person DATE.
That's pretty much it - it's really, REALLY easy to keep a girl's interest if you just listen to the above 3 things, and yet people still have a hard time grasping this concept.
But fine - let's look at where you went wrong, shall we?
1. She texts you after the date - and then you got into a convo with her.
Doesn't seem like it would be a bad idea, right? After all, she initiated it, which is what we want them to feel like doing. However, YOUR mistake was (a) answering her back right away, and (b) not setting up a date for 3 to 5 days later during that convo. You need to be setting up DATES anytime a woman reaches out to you, and either ignore or significantly delay your response time, then say "Hey, I'm busy now but I'll see you on our next DATE." You communicate with her on the DATE, not in between dates.
2. You text her, and she seems enthusiastic
This is another mistake guys make - they see all these emojis and LoLs a girl sends to them and assume that's how she actually feels. Guess what, it's not - here, I'll prove it:
"Hey guys, guess what? My dad has cancer! I'm really sad about it, lol! :cheer: "
See that? I'm "happy" in my message, while feeling something else entirely in person. The reality is, her subconscious is actually NOT happy that it's hearing from you so frequently because (a) it's not giving her brain the time it needs to miss you and build up interest level, and (b) it's starting to make her less interest in you because you're taking part of the MYSTERY away from who you are. Specifically: she can't wonder about what you're doing or how available you may or may not be when you're readily available to talk to her at any time.
3. The next day you text her...
...and this is the NEXT mistake. Back-to-back texting days? Only if she's the girlfriend - if she's not at that stage yet, YOU initiating text messages everyday is a HUGE no-no, and one that is likely to cause her to become disinterested.
4. You text her AGAIN at midnight...
Stop. Just stop. Really - anytime after 10 or 11 pm, you should just stop calling/texting anybody unless (a) you already have plans set up, or (b) she's been flirty with you all day and you're trying to set up a booty call.
5. You have a dull boring convo...
...because text message convos are ANNOYING. And she doesn't want to keep talking to you on the dang phone; she wants YOU to ask her out so she can talk to you IN PERSON again. And plus - it's MIDNIGHT!! I tell my girlfriend not to call me after 11 or before 8 AM 'cause I'm TIRED and will be asleep! Maybe the convo looked boring 'cause she was trying to friggin' REST.
6. You asked her out for the next day...
...which is a "last minute date," which you don't set when you first start dating someone. You have a better shot at getting a date if you actually set up a day and time a few days in advance, with an ACTUAL date plan vs. "hey, we should do something tomorrow" (something = not having definite plan for her to get excited about).
7. It's been 4 days since she contacted you...
...because YOU are the man. And YOU are supposed to contact her. And not with random text, or "rapport building convos," but to ASK HER OUT ON A FRIGGIN' DATE. And when you do, you should be CALLING her, not texting and asking her out like a Beta would.
In short: your way of communicating with her is what's messing this whole thing up. Does this mean she's not uninterested? Maybe, maybe not. BUT, at the very least, you could find out by thinking of an activity you two could do 3 days from now (Sunday), then CALLING her to ask her out. Depending on her answer, you'll know if you have a shot still or not, but CALL her so you can tell, by way of her vocalizing an answer, if she sound excited, hesitant, or not interested. And for any future girls you date, STOP TEXTING THEM and ONLY CONTACT THEM FOR DATES.
Hope this helps