Git push rejected "non-fast-forward"

It looks, that someone pushed new commits between your last git fetch and git push. In this case you need to repeat your steps and rebase my_feature_branch one more time.

git fetch
git rebase feature/my_feature_branch
git push origin feature/my_feature_branch

After the git fetch I recommend to examine situation with gitk --all.

try this command

$ git push -f -u origin <name of branch>

i.e $ git push -f -u origin master