1998 Ford Explorer help!

I was on my way home yesterday. Rounded the corner on the street I live on an when I tried to pick up speed my explorer wouldnt move. I had to push my vehicle the rest of the way home. I had a man look at it. I had my vehicle in my driveway put it in drive and mashed the accelerator. The engine reved up but my vehicle didint go anywhere, it wont even go in reverse. I got the man to change my transmission filter and fluid. I got it running but when I drive I have to drive my vehicle like it is has a manual transmission. I have to switch it into 1st gear to start off, then go to 2nd at about 10mph, and when it gets up to 25-30mph I have to put it in drive. My explorer has an automatic transmission. What is wrong and what should I do?