I know cancers don't keep people around in their circles who don't really matter to them. When someone does make an impact though, why can't you let go of memories or the idea of someone particular go?
Don't be the home wrecker, no one likes that type of girl. Have more respect for yourself and for the others woman and their children. I think fire signs tend to be okay with being the 3rd party or mistress. I don't agree with this, so you should leave it alone.
Wait and don't think about the timing, it's going to eat at your thoughts and change your moods. My Cancer ex took me over about 4 months in but he said he was ready and then I later met his children 5 months into it. It was soon! I miss him and I hope we come back to one another.