connection to MongoDB via env variable returns DB_URL is not defined

I am trying to connect to my MongoDB database using a env variable that is in my config.env file so that I don't leave my connection string visible for everyone, but I keep getting the error that my DB_URL is not defined, why? How should I do it? I installed env2, then I created my confing.env which looks like this:

export DB_URL='mongodb://admin:username@password/heroku_url';

and then I required it in my server js which looks like this:

// Requiring the dependencies
const express = require('express');
const app = express();
const bodyParser = require('body-parser');
const cors = require('cors');
const mongoose = require('mongoose');
const PORT = process.env.PORT || 3001;
const itemRoutes = express.Router();
let Comment = require('./comment.model');
require('env2')('../config.env')

app.use(cors());
app.use(bodyParser.json());


mongoose.connect(DB_URL, { useNewUrlParser: true } )

const connection = mongoose.connection;

connection.once('open', function() {
  console.log('Connection to MongoDB established succesfully!');
});

// Serve static assets
if(process.env.NODE_ENV === 'production') {
  app.use(express.static('build'));
}

itemRoutes.route('/').get( async (req, res) => {
  let collection = connection.collection("posts");
  let response = await collection.find({})
  .toArray();
  res.send(response);
});

itemRoutes.route('/comments').get( async (req, res) => {
  let collection = connection.collection("comments");
  let response = await collection.find({})
  .toArray();
  res.send(response);
});

itemRoutes.route('/userComments')
.post((req, res) => {
   res.setHeader('Content-Type', 'application/json');
   let comment = new Comment(req.body);
   comment.save()
   .then(comment => {
     res.status(200).json({comment})
   })
   .catch(err => {
     res.status(400).send('failed')
   })
});


app.use('/', itemRoutes);
app.use('/userComments', itemRoutes);


app.listen(PORT, function() {
  console.log('Server is running on' + ' ' + PORT);
})