Mate it shouldn't slip out of gear.
Something has come out of adjustment in your throttle, and gear selection linkage, Maybe just caused by the cable stretching over time, or a kink, dirty many reasons, even just a lube can fix it.
So for a quick check put in neutral and spin the prop if it clicks, that linkage will need to be adjusted So it doesn't wear the dog gear.
Also the throttle linkage see if you can close the throttle more by just moving the linkage, if you can that be why the idle is up.
It should be a easy job 2 mins.