Be sure you aren't having feelings for 'bob' out of boredom, or like a safety net. He was safe and kind when 'joe' was around, and you were messing him around.
I understand that when you're young, relationships can be a little bit fickle, everyone's learning. However, 'bob' has obviously learnt from your behaviour, and doesn't want to stand for it. Which is good in some ways, believe it or not.
Before you tell him something/or even try to about how you feel, consider some things,
do you really like him, enough to respect him enough not to go off on a whim with joe or any other guy again?
Or is it just boredom or loneliness thats making him sound so appealling, and not real interest. Maybe when you have him again you won't want him again?
If there's a good chance you feel you may do to 'bob' what you did to him before, respect him enough not to make him go through all that again, and don't even go there.
But if you do feel you can be good to 'bob', then go for it, but make no mistake in the fact you've probably got some hard work ahead of you making him trust you.
And what to say? a genuine 'Sorry about the way i treated you, you didn't deserve it' might be a good starting point!
Sometimes when people don't apologise, and don't even realise they haven't is the thing that hurts most, like they havent acknowledged the pain they caused you. Even though apologies sometimes feel like 'just words' its better to apologise and mean it than to not bother because you think it'll make no difference.
I hope it goes well, whatever you choose to do.
Damned ladies of Orpheus
Your arias cause a stir in my sad
Sad and lonely heart