Problems with transmission

The first issue was it not wanting to go into gear after a stop I couldn't even get up to fourth after putting in some transmission fluid. Mind you there was never any lights going on until that first issue to check engine light came on. It had transmission fluid put in the rest of the bottle it took it in. Afterwards it happened to be worse; now it won't go into any gear in fact it rolls forward when it's in reverse on an incline. I'm not sure whether I should replace it with all the hassles that'll go with it and cost vs selling it. It's still a good car with low miles for the year. I don't know where to look to see how I should price it or if that's a better option. My transmission type is eyn.