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I have a 2002 Honda Civic that has started having troubles shifiting from park into reverse/drive. It stays in park just fine. It started where it would shift into reverse and drive just fine, but then when you would accelerate the first time, the car would make noise like it was accelerating but it wouldn't actually move like it was accelerting (it was at the very least moving forward.) Today, I went to drive it and the car acted like it was in neutral, and would only roll downhill. It did seem like when it was on flat ground it had a tendency to roll forward, whether it was in neutral or drive. I shut it off to do some research, then turned it back on and let it get to running temperature before checking the ATF. That checked out fine. I jumped back in the car and put it into reverse and it worked perfectly. Any ideas where to go from here? I am fearing the worst(transmission is going) but wanted to crowdsource some other ideas of what it could be. I feel like I got very lucky for it to finally go back into reverse. I have a very small amount of knowledge on cars but can always follow directions or youtube videos.

 
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Trans could be on the way out. Try draining and refilling the trans 3 times to see if that helps. It won't cause any further damage. You can use valvoline maxlife instead of oem fluid if you want to save a little bit.

 
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​​​​​That'll be my plan. Any advice on how many miles between each draining?

 
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Enough to get the fluid warm. You can even do it on jack stands.