I am not an expert by any means, but I did some research about interstitial pregnancies when my doctor expressed some concern about where my current pregnancy was located.
What I found when I researched this: management of interstitial pregnancies is more difficult than tubal pregnancies, because the embryo can implant more fully in that tissue. Some interstitial pregnancies require just a D&C, and others require a D&C plus a resection. Part of the management strategy depends on where exactly the pregnancy is located.
From what I have read, pregnancy is possible after an interstitial pregnancy, but you will need to be closely monitored during the pregnancy, and will probably require a c-section to reduce the chance of uterine rupture during delivery.
Just so you know, though, doctors can be wrong about the location of a pregnancy. I had a very early ultrasound (5 weeks, 1 day). It took my RE a bit of time to locate the pregnancy because it was much further to the right side of my uterus than she expected. She told me that there was a chance that the pregnancy was interstitial and might require surgery but that she couldn't tell for sure (yet) where the pregnancy was located.
My next u/s was a week later, and my RE found the pregnancy smack dab in the middle of my uterus. I really don't know what changed (and I googled the heck out of this) but everything has been fine since then. I don't want to give you false hope, but there is still a chance that your pregnancy could be viable.
I am really sorry that you're going through this, and I really hope everything turns out OK.