Honestly, why is it so hard to keep our expectations low, even when it’s just about texting and no dates yet?

Example:

I’ve “known” this guy from online dating since last December. During that time he asked me out twice but since I wasn’t ready I gave him two excuses and texted him sporadically. He always texted back.
For the past two weeks we’ve been texting a lot, like every single day. From my perspective that seemed like he was interested and making an effort. Also this time I was ready, I even told him that I was taking sometime away from dating and he said he understands and respects that.

So yesterday I asked him out and he replied “when it’s good for you?”. When I suggested Saturday night, he changed the subject and still kept texting without giving me an answer.

A normal person would just say “wtf” and leave it be, but for me, I looked at it and started CRYING. It felt terrible, like rejection was already there. I felt extremely upset, put my phone away and cried in bed for fifteen minutes. 
Why does it have to be so fucking hard even if you don’t even know each other in real life? I feel like a crazy person for having a reaction like this towards the situation. I feel like I’ll never be able to have anything healthy with anyone when it comes to romantic/dating/having a SO and etc.
What do I do to control this? What do I do so I don’t get extremely upset in situations like this one? Is there is anything I can do about it?

