G'day Mate,
That's where an Instructor comes in, he/she will trim the plane, so it flies slow enough for you to control it, usually straight & level at half throttle, there is no substitute for experience, with this addiction, um I mean, hobby.
I'm not gunna say anything about what you did, I'm just glad you now realise, that we do know what we are talking about. It is a bit harder than it looks, this flyin stuff. EH.
And one more thing, the 40 size Boomerang is a great plane with an OS 46AX, Bloody GOOD Choice.
Well Done.