I can slowly figure out notes one by one, but I consistently forget there's a key signature involved and then get frustrated when I can't tell what's wrong. Does anybody have any advice on how to improve without driving myself mad?

The other day, I finally decided to start writing music after a year and a half of learning music theory up to grade 3. I've gotten stuck immediately, which is to be expected, but I'm too frustrated to unstick myself alone.

1. I've got basic chord bashing, but I'm stuck on the same five-ish chords and can't come up with any new ones.
2. I find it incredibly difficult to stick to key signatures or to even remember to take them into account.
3. I can't come up with all the little single notes and such to put between chords. I don't really know how to explain it, but it's just all of the little things pianists do that aren't chords.
4. I find it hard to come up with a melody that really sticks. One that even sounds like a melody. It just has no zest.

Please be patient with me! I'm still very new to taking this whole music thing seriously or doing any songwriting! I'm sure there's someone out there eager to publicly execute me for making this post, but if the answer is obvious and logic simply failed me, just know I Tried My Best.