I sympathise. I've flown with congestion twice, and the descent is not an experience I would like to ever repeat. Here are some tips to avoid the splitting ear pain, both pharmacologic and non-pharmacologic.
Try to get hold of a nasal steroid, like nasonex or flonase at least two days before your flight. These can take about 24 hours to start to work. If you have some residual congestion on the day of the flight, make copious use of nasal saline flushes. If that doesn't work, go for broke, and try to dry everything up with a decongestant, like a medication containing pseudoephedrine (check the label to make sure this is the active ingredient- in much of the US, products marketed as "Sudofed" contain the much less effective drug phenylephrine) or afrin spray. These are not usually recommended in colds, because they tend to prolong the duration and have other side effects. But if they work, they will save you a very painful hour during airplane descent! Benadryl (diphenhydramine) will do nothing for the congestion unless it was caused by allergic rhinitis.
Non-pharm options. As you descend, if your ears start to hurt, try to mobilize some of the fluid out of the eustaschian tube by vigorously massaging the area of you head just anterior to the ear's tragus. You can also try the valsava maneuver- puff up your cheeks, pinch your nose and push out like you're blowing your nose. Keep trying every few seconds until the eustachian tube opens.