They say it takes 2 years to completely get over someone that you truly loved... more or less depending on how close you were to them and how you are. It might be just too soon to move on for you. It is somewhat unhealthy to do the rebound thing, it usually doesn't last long because you are still in love with girl A.

Get over A and then look at getting back into a long term relationship, it doesn't necissarily mean you have to wait 2 years but you really have to put her behind you and get it out of your mind. I think a sign of when you are over A would be when you can see her with another guy and not be jealous. Its something you're gonna have to work on man, every single time you think of her conciously remind yourself that you are DONE with her and she is in the past, don't let your mind dwell on her, its unhealthy.

As for B, you should explain that you just got out of a relationship and you don't want to do the unhealthy rebound thing. Ask her if its ok if you two go slow at first untill you're sure your heart is healed, explain to her that you just don't want to end up hurting her. If she isn't willing to go slow, don't sweat it man, there are pleanty more fish in the sea.

Hope this helps,
Tennyson