1996 Cadillac Seville STS has a slipping transmission problem

I bought this car a couple months ago and recently OD started slipping intermittently. I drove it for two days like this and kept an eye on my speed so I didn't get to OD. I checked fluid and it was low, so I put some in. I ended up putting a pt too much in. I don't know if that made things worse or if it would have slipped more anyway. So, I drove it one more day and all the other gears were now gone. I have first gear, reverse and park that work. I have been told that I just need a fluid flush/filter change, but I would like some input before I pay to have it towed somewhere. I have not drove it at all since then. I also wondered if I syphoned out some of the fluid and got it back down to "full" if maybe that would fix it. Any input would help. Thanks.