2001 Honda Odyssey Putting van in drive

Transmission problem
2001 Honda Odyssey Front Wheel Drive Automatic 148000 miles

My question is this, I have been having a problem for the last week putting my van in drive. I put my foot on the brake and try to put the van in reverse and I cannot get it to go unless I try 5 or more times. It will go in gear after a few times, but I think I may have a serious problem coming on. I hope you can help. Thanks
Michelle