This is going to sound clinical, but it's a really important aspect of songwriting, and a lot of songwriters get very confused about it. Ready?
You don't put emotion INTO your writing. Instead ...
You try to create emotion WITHIN your listeners.
So, if my lover has just dumped me, and I try to write a song right there and then, I'll probably come up with rambling, teary rubbish (might be worth jotting down a few notes, though).
But if I live with those feelings for some time, find what's universal in them, and express them with specific, original imagery, I could be on to something.
Here's an example of this, from Tom Waits.
The song is 'Kentucky Avenue', and I'm afraid I have to spoil its ending to show how it works.
One kid is inviting another out for an evening's worth of high-jinks, and listing all the mischief they'll get into. After three and a half verses of great, specific language, from Buicks to strip poker, Waits does this:
I'll take a rusty nail and scratch your initials on my arm
And I'll show you how to sneak up on the roof of the drugstore
Then he does this:
Take the spokes from your wheelchair
And a magpie's wings
And tie 'em to your shoulders and your feet
I'll steal a hacksaw from my dad
And cut the braces off your legs
And we'll bury them tonight in the cornfield
You, the listener, don't know until this point that the second kid is in a wheelchair, with legs in braces, and that none of these plans can really happen. It's all hopeless bravado, I'm a puddle, and Tom Waits is a canny bastard.
I suggest you look at how Ed Sheeran, or similar artists, make you feel things. Look at their individual word choices, melodies, chord progressions, and aspects of their performance. Then try your own version of those approaches, in your own writing.
And remember, Ed might have been weeping tears of pain as he wrote, but chances are he was chuckling at what he hoped to do to you.