I would pull the trans pan for a look see. Check linkage hook up inside the pan.
Since you have little experience with a transmission , I would suggest you find someone with trans knowledge to help.
If the trans shifts in & out of park , I would suspect there is an internal problem causing the 'reverse only ' situation.
Flushing fluid will not help this trans
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.