Took a trip to Birdling's Flat (small rocks and pebbles) and did some driving. Noticed when I got stuck that I didnt have much power in reverse. What did I do then? Give it more throttle of course

Now, When I start it in the morning(COLD). I put it in drive, Reverse will BARELY work as long as I stay off the throttle. I put it into drive and accelerate. It does not shift into second unless I move the shifter into second gear and then it shifts. I then put it back into drive and it shifts into 3rd.
Checking the fluid, It is Blackish and thin. It was waaaayyy too full so I emptied about a liter out and its still a bit over the full line when hot.
I am a novice when it comes to vehicle repair. I have heard suggestions of it being a clutch pack. I also heard it could be a solenoid.
Any suggestions? Thanks in advance:)