After having read all the responses, I think I'm in the middle here.
Also, try not to regret your Trek purchase. If you had bought a road bike, you might be STILL thinking you made a mistake, and wished you had gotten a hybrid or something. Very few people are happy with one bike, and those who are have likely gone through a bunch of them to get there.
If you had a road bike, you would probably be fantasizing about something that didn't have such a jarringly hard ride. Those thin road bike tires are often run at 120 psi; rock-hard for low rolling resistance. The penalty is a hard ride. Since road bikes are counting every ounce, to once again help with easy speed on smooth pavement, they never have suspension either. The seats are made for light weigh, and are also hard. You lean way over, so you have to look up to see ahead of you. (neckaches) You have a bunch of weight on your hands and wrists, which leads to problems with these areas of the body in some folks. I have to move my hands around the bars all the time on my cyclocross bike to not be in pain, and it is not even a proper road bike!
By your signature, it seems that you've lost a lot of weight with that bike, and I think you deserve a little something to keep you going. My advice is to treat your Trek to a little TLC, but without going crazy and trying to convert it to something else. Here's what I'd do:
- If it has a flat bar and you're thinking about drop bars, why not compromise by adding bar ends? That'll give you another hand position to switch to, which can really help.
- Treat yourself to a sprung Brooks saddle. A B67 seems about right for your bike and you will thank yourself. I have one on my TriCross, and it has doubled my cycling enjoyment, easily.
- If you don't have them, get a good set of padded, fingerless biking gloves, LED lights, fenders, and a rack.
- Consider upgrading the pedals to combination flat/clipless and maybe a pair of clipless shoes
By doing these things, you will have customized your bike to be the best commuter it can possibly be, but without launching into a frame-up conversion. (bar ends and saddle should be first priority, IMO.)