You're too young for the 7 year itch!
Most people choose to live in long term monogomous relationships. I agree, they tend not to last forever.
Stonheather you've had many difficulties with Greg, maybe too much damage has been done to repair the breaches of trust in your relationship. There's still something there, though. It's not long since you split up and you chose to get back together. The best thing to do is try and get to the bottom of what it is that's bothering you so.
You know he's a good guy yet you don't see him that way. Try and decide if your current impresion of him is accurate or whether you know the good guy is there under all the crap that's got in the way. If you decide the relationship is worth trying to save, tell him why you now see him differently and let him do something about it, if he's able, rather than punishing him for it.
Either way, take things slowly, make no hasty decisions. You may need Greg's help for some time yet. You are pregnant with his child, you are entitled to his support even if you know things will not work out in the long term.