It's very understandable. We all adapt, sometimes in ways we don't fully realise. You have been head down, doing the treatment, concentrating on the here and now, and suddenly it seems like there isn't any here and now any more.
My oncologist warned me I may feel like this after my last Herceptin - I didn't but what he said made a lot of sense.
You are in for (sorry!) another period of readjustment. This is a good one - it's readjusting to NOT having treatment, not looking for a vein, not having a diary full of appointments (well, not so full of medical ones anyway). You may feel a bit adrift for a while as you establish new routines. If you work, that may help. If not, now is a really good time to establish some new must -do's - go to the gym, walk the dog, take up a hobby, make lunch dates with friends.
Your anxiety is about not battling - but you really have earned a break. The feelings will ease, and then you can enjoy what the last 18 months has all been for - feeling you are better and that cancer is behind you.
very best wishes