Chemistry is important for the longevity of a relationship. If it's not there -- and you don't think you can develop it -- then you're will probably have a difficult time maintaining interest and being fulfilled in the relationship.
Think about why you're still with him. Are you settling? Does it make you feel "safe"? Are you avoiding being alone? If these are the reasons you're with him, it doesn't sound good.
Decide what's going on with your current relationship before you consider anything with the ex.