I have the same sorts of problems. But I find my best lyrics come from when I don't have much of a subject at all.
When I write, I normally start by going over the catchiest bit of the song. I'll play through it and end up humming a melody. Ultimately a couple of random words will come out, I'll quickly jot them down and it gives me something to go on.
Personally, I find if you write down whatever ideas come to mind, it makes the process a lot easier. I spend far too much time writing ideas off in my head, and that ends up blocking ideas.
On of my favourite set of lyrics I've written ended up being full of big words. I didn't have a theme or subject in mind, but I found one began to form as I wrote sentence by sentence.
More recently, I've tried to relax a bit more, and honestly, not be afraid of being corny. If it works well with the music and fits well, it can't be bad! Think about how many successful musicians out there who have very simple, cheesy lyrics, but have great songs because of it!
Adele for example. "Maybe I'll find, someone like you, I wish nothing but the best, for you. Don't forget me, I swear, I remember you say, sometimes it lasts in love, but sometimes it hurts instead."
How huge is that song?
I know if I wrote that, I'd scrunch up the paper and toss it away, yet I'll happily sing along to it, and so will millions of others.
Regarding rhymes, it can help or hinder. More and more songs aren't rhyming so much these days. Again, if it works with the music, who cares.
But, I used to use rhyming a lot to help me write lyrics. I'd come up with a line or something, then I'd pick a word to rhyme with, use rhymer.com to find some rhymes, sometimes double check the meanings in a dictionary, or see if they had second meanings I didn't know about, and I'd find my subjects would come through that. Then I'd use a thesaurus for some of the words that Rhymer gave me, and it would maybe help me with a different part of the song.
I find song writing hard too, but I think the thing that makes it hard is overthinking it. If it makes sense in itself, and flows with the music, you're set.
Just make sure the chorus is reasonably catchy, and you're set. You could even boil down to a simple phrase. I recall hearing a song by Pixie Lott several years back called 'Broken Arrow', from what I remember, every lyric surrounding that was very plain and simple, yet the way she built it up lead to 'Broken Arrow' being a catchy couple of words that made the song come across as vaguely deep.
I don't really like pop music all that much, but if you want some freedom to lyrically relax, pop music has some of the simplest lyrics ever!
Edit: And another thing, the more you read over your lyrics, the more you will start to hate them. I can't tell you the amount of times I have read, and read, and read my lyrics only to end up scraping them. Things become overly familiar to you, and you will start to pick out or make up problems that aren't there. If it's good in the heat of the moment, press on! Sing along with it, and see if it works with the music when you've finished them.
I find the same problem when I'm building tracks in my recording phase. Recently, I finished a track called 'All Our Lives', as I mix/record/produce myself, I had to listen to the song hundreds of times over, and listen to little bits of it over and over. I end up hating everything about all of my songs, but finish them anyway. I end up not listening to them for months and months, and then when I go back, I realise why I finished it, because it was better than I thought at the time.
Somtimes were are our own worst critic.
Hope there's some helpful stuff here!
