There are books on this, as you say - so complicated I wonder if even the author could figure it out.

I tried those and couldn’t handle it.
What you’re doing is ok, but it’s not really mental prayer and you are doing good to want to grow in that pathway. Mental prayer is thinking, yes. But it’s thinking about God in some way. It’s not just thinking about what you want to say.
The most important thing (St. Teresa teaches) is to start with a book. Spiritual readings, meditations, writings of saints - any Catholic spiritual topic is fine. Short meditations are probably best and easiest.
- Read the meditation. You don’t have to read it all, or maybe you have to read more.
- Read until a thought strikes you. Maybe it’s the first thing you think, maybe something more.
- Stop reading. Keep the thought in your mind.
- Now - think about it. What does it mean? What does it tell you about yourself? About God? about Jesus?
- Turn those thoughts into prayer:
– “Thank you, Lord - for such a good thought”
– “Lord, help me to understand this. Help me to change my life like this”
– “Lord, I praise and worship You for your greatness and for being so kind to me. Help me to be a better friend, disciple, person …”
Lots of things like that. In other words, you use the mental thoughts you gained to pray better.
Reading a book on the life of Jesus would be great.
Another method, not just words in your head - but using visual pictures in your mind.
That is the mental prayer of the rosary - or anything on the life of Jesus.
The Birth of Jesus. You picture it in your mind. See the stable. See Mary and Joseph. Watch the shepherds coming from the field. It’s like a movie. Then do the mental prayer. How poor and isolated the Holy Family was? How amazing that God actually was born on earth? Etc.
Finally - the thing you also want to do - after all that thinking and talking with God - is take some minutes to be totally silent in your mind, with only the thoughts of praising, thanking, worshiping and glorifying God. With quietness, God will give you His inspirations.
Closing thought - you could use a book of Catholic artwork. Let the image go into your mind. Then think about it. The glory of God, the power of angels … etc.