Sorry you are going through this. I'd say first that you need to exit your current relationship. So not fair to hang onto a guy that you wish were your ex and your ex were him. This just will never work and it's unkind to keep him stuck with someone who doesn't love him the way he deserves. So, do the right thing there and let him go. It is okay to be on your own, I promise. And being on our own will make you stronger and better able to pick a good partner in the future.
It's natural to have regrets and at times longing for an ex boyfriend. Six years later seems to me though to be based on a kind of false idea. The thoughts of him are a relfection of your current unhappiness perhaps.
I would let your current partner go and possibly do some therapy to understand why you are sort of 'stuck' yourself in the past. The idea and thoughts of an ex 6 years ago might be you trying to put a band aid on some things currently going on that you're not content with.
Again, I'm sorry you feel this way. And I hope you can get past this soon so you can start living happily in the present. good luck
Before I can even answer this question I need to know a few more facts. Why did you and your ex split up?
Do you have any children together? Do you see him often or not? And have you cheated on your new boyfriend with your ex?