I think you said it all when you said "I love my ex". You didn't say you loved your current boyfriend, you said you had feelings for him. I would think it's kind of unfair to the boyfriend to keep him on the hook when your real feelings of love are for someone else. I mean, you wouldn't want either one of them to do that to you, right? Especially if you felt love for them. I think your best bet is to just tell the boyfriend that you're having a tough time with feelings for the ex, and that maybe it would be a good idea for you to not have anybody special in your life at this point, and then take that time to really sit and think about who you really want. Sometimes you need a clear head for decisions like this.
'Follow Your Heart'
You need to follow your heart. You also need to think of some things. What caused the relationship to end before? Will it happen again? What will make it work this time? What has changed with my ex/me since the breakup? Try not to make the same mistake twice. Don't waste time trying to fight for something that is already lost. And furthermore, if your present lover cares for you, would you want to hurt him by leaving?
You have to make a list with what you liked and disliked about your ex and what you like and dislike at your boyfriend now. Then try to think about yourself. Were you happy with your ex? Did you feel safe, comfortable and desirable with him? Why did your relationship end? Did he change in the meanwhile? Did you change? What do you expect from your man? What can you receive from these two guys? Sex and good intentions are not enough. You have to think a bit about your future, your life and the direction you wanna go. Your ex can want you now, because this will be a personal victory, to claim you back. But how will it be after? Will he trust you? Will he forget that you came back from another relation?