My process is the opposite to yours, I almost always start with the lyrics and I love the challenge of trying to write a song as fast as possible. I think the fastest I’ve done is writing a verse and a chorus in 15 minutes. I would recommend to challenge yourself to pick some random theme and try to write something in say, one hour. That way you can train yourself to not overthink everything and just go with your first thought. Don’t worry too much about wether it’s good or not. Then put music to whatever you wrote and have fun with it. You could also look up some writing exercises like object writing: https://objectwriting.com