<human>: Can you condense the text into three sentences or less?

I am at a loss here.
You are not supposed to be changed by or to want to change the person you're with.
Also. I have been told that there's no such thing as love at first sight, and that you should give the other person or the relationship some time to know if it will be "the one".
But then there's this thing where you shouldn't keep people in your life that don't do you much good and where, if you look at a relationship in retrospect, you spent waaaaay too much time waiting.
Then, how do these things balance out?

I (22 yo) am dating (6 months) a guy (21 yo) for the first time since my ex (27 yo, 3 yr relationship) cheated on me with his best friend while we were living together (and denied it all along by telling me I was crazy and that it were my issues making me doubt him for no reason). That has left me in a very strange position, where I really don't know anymore whether to trust people and open myself up to others or not.
The thing is, when we are together he seems really nice. I mean, all the "signs" of him being sort of in love with me are there. But then, when we're not, it's completely different. He rarely has time for me and only meets me when he's made the plans. If I invite him somewhere he just says "Alright, we'll see later" and never gets back about it. 

Sometimes it would appear he's really shy, since when we began dating he didn't take initiative for anything. And even now, i.e. I sent him a bikini pic the other day and he just commented on the pattern the fabric had.
I don't know if it's my insecurities showing off or he's somewhat shy, or simply he's not a keeper.

Sorry for making this so long, it's the first time I've been able to put it into words.
<bot>: He looks and talks and acts nice but only when he decides to. Could he be shy? Or is it a waste of time?