You have moved on. Don't go back over old ground. You can end up being 'used' and spat out and burnt in a very horrible way if you are not careful. But will be the cause of this by your own actions and you do have choice.
You may have had 'feelings' for your ex but he doesn't have those same feelings for you. Your ex probably still views you as a useful 'possession' or as a 'conquest' So if he didn't have those feelings for you then he certainly doesn't have them now no matter what he says.You are being used. Make sure you know the difference between someone who wants to give you attention for sex and someone who has your best interests at heart. Not just attention to feed their own selfish needs. Your ex has come crawling back because of jealousy and wanting sex. You should be fair and consider how your new guy feels when you swap your attentions back to your ex. Be absolutely clear you have moved on and your ex has no place in your life. Your ex won't have changed. After messing you around he will do just the same again and leave you just as he did before. Stop flip flopping. If you don't find someone who sets your heart on fire don't have a relationship them just for the sake of it. Don't be yanked along by someone like your ex who just wants to screw you over, take what they want and then clear off. Do you wonder why that makes you feel like crap? You alone allow that to happen and you have a choice to say 'no'
You probably enjoy the 'flattery' and the 'attention' from your ex but you need to be sure of what you want here? What does a 'good' relationship look like for you? The worrying signs are that you are with a boyfriend "just because he treated you so much better than your ex?" Read what you have written. How low is your bar? What is wrong with being with 'no one' because you haven't met the man of your dreams? The feelings of wanting to be with someone should be massive, of a strong attraction, and those feelings should be returned by the person you are with. You should not just be with someone because they are convenient or 'nice' - a little bit like your ex being with you for three years? So if you have a massive attraction with your new guy stick with it, work with him and enjoy your time. Be sure you have feelings for your new guy, but don't go back on your ex who just wants to flit in and out of your life, treat you like a piece of shite, take what he wants and to clear off again. Maybe that is what you want because you don't value yourself? You deserve better and so does the new guy you are with right now. Your choices dictate the quality of your life and of your future happiness later down the line.