I also have this big problem of overinvesting when I DO get a date, and expecting too much or being majorly disappointed if the guy isn't into me enough to want a third date..
The thing is, we have a CHOICE... to override our "natural evolutionary" instincts. Some people choose to let their minds rule their primal urge to reproduce, in order to pursue a monogamous relationship, because in the long run they know that will make them happier..
One time last week he sent a text apologising to me that he forgot to ring when he told me he would ring one day. That really surprised me. Was not expecting to hear from him at all. Then he asks me if I am doing anything I let him know I am free for a chat and then rings me not long after. This all being on a week day. He always initiates. He has rang around three or four times to just say hello. All those times were during weekdays..