1. backup your sources.list
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2. change repos to bionic
sudoedit /etc/apt/sources.list and replace trusty with bionic. Probably some source lines are not the same for trusty and bionic so if you still have errors for them you better remove these repositories from your sources list, the safest way is ...
It seems that you've upgraded python3 version. As explained here https://askubuntu.com/a/1191220/607679; because of the python3 update, usr/bin/python3 points to a python that does not have apt_pkg.
You can create symlink like below. This should your problem.
ln -s /usr/lib/python3/dist-packages/apt_pkg.cpython-36m-x86_64-linux-gnu.so /usr/lib/python3/dist-...