I wouldn't say I'm "frustrated", I feel pretty decent about my writing, but I do wish it was easier for me. I think songwriting changes from genre to genre a bit, typically at least, I'm not going to say absolutely. Regardless, I'll share some things that work for me or what I kinda live by.
- I'll start off with what @jfrench said. Keep Going. I think its especially important to keep in mind when youre writing and you feel really good about the bast few bars but you feel stonewalled. At that point, grab another sheet of paper, Forget any expectations you have for yourself and just flow, just go, get it out, all while having a vague direction being the idea of feeling you're trying to invoke.
-If you ever feel discouraged and feel like youre just not creative enough or any of that crap, try something thats just more fun. Put your main focus aside, find something fun to write about. You can do this by checking out "writing prompts" you can use this in song form, prose, or write a short story. For example, one of my favorites is "Imagine you're in a field, or forest, or mountains or what have you, and imagine you see a door that has no business being there. What's it look like, what's on the other side?" Thats a fun one to reuse and get you feeling creative again, get that confidence then go back to your focus when you feel good about your abilities again.
- If I'm writing in rhyme (usually I am, I like rap) but feel stuck like my flow is starting to get weak and I'm forcing rhymes, I'll start just writing in prose, I forget about the rhyme or the flow, I just write in prose trying to grasp the emotion or feeling that I'm working towards. Usually through that I find another way to say what I'm trying to say, then I get to draw connections and thats a really fun thing to do, also I think it shows more mindfulness.
-Know when to go into "open" or "closed" mode. I get this notion from this excellent lecture: by John Cleese of Monty Python. When your open, your receptive to influence, your mind is loose, just gathering ideas, letting it come to you, dont force anything, sometimes you gotta stick with it longer, then when you get ahold of something inspiring, go into closed mode. thats when your focused, tunnel vision, frantically write down that capture that inspiration. when you get roadblocked again, go back into open mode and fish for some inspiration.
carry a notebook always. I hope you do this already. any type of writer will tell you that this is a must. I'm always writing down words or phrases or ideas or just a few bars. Just last night I was watching looney tunes (yes I still watch looney tunes) and in the episode "Baseball Bugs" the "bad guy team" was named "Gas-House Gorillas". Thats an awesome word and would be really fun to drop that reference in a song so I wrote it down haha. It's a goofy example but trust me, having a word/phrase bank is super helpful.
Final note, if you're feeling like you head is spinning and cant get a grip with your writing, go back and copy someone else. One think I really like doing is finding a song, or poem, or hook or whatever that I love, copy it into word, add extra space between each line, then write a parallel story using the completed material as the skeleton and write your lines underneath the original. I feel like this really helps your sense of direction.
So if you're frustrated, keep trying different things! find what works. Dont limit yourself to just "songwriting" get comfortable with all sorts of creative writing and use all those elements together when it comes to writing that song youve been trying to get out.
Footnote: this probably only partly relates but I'm such an advocate I'm gonna plug it anyway. try Freestyling! Im awful at it and I don't care its really fun and once in a while I surprise myself with a really cool simile or metaphor and then I write that shit down. It really boosts your creativity skills.
If you're looking for feedback dont hesitate to ask.
EDIT: dont know how my post ended up with sub bullets. interpret them all there own thought
2nd EDIT: I just remembered another tip I keep in mind. Build confidence, it means a lot. There's a reason why Tiger Woods sinks 50+ 10 foot puts before a tournament or why basketball players begin warmups with simple layups. If you're struggling to write a song, go back and sink some putts for confidence, just write 2 juicy lines, or a hook. Then you feel good and come back to your main focus while also building that handy word/phrase bank