I am not an expert on the subject, so I contacted FamilyTreeDNA, due to your issue on the mtDNA sequence. This is their reply :
Thanks for contacting FamilyTreeDNA. At the full sequence level, even an exact match to your DNA (so a match with a genetic distance of zero) would likely share a common ancestor with you on your direct maternal line anywhere within the last 5-16 generations, or 125 to 400 years or so. When you start looking at matches with differences (genetic distance of 1, 2, or 3), those common ancestors can be even further back in time. Once you're looking at matches that are only at the HVR1 and HVR2 levels, it's possible that you could share a common ancestor with those matches anywhere within the last 1000 years.
Depending on the extensiveness of your tree and your genealogy research, it's possible that your common ancestor could be further back than you have traced your genealogy.
The article I've linked below has a ton of great tips for getting the most out of your mtDNA matches and finding your common ancestor with them. I strongly recommend reading through it.