Almeter and Bernard -
Congratulations on your retirement! Now it's time to have fun….!
First off, I can imagine how big this may look to you as it looked very big to us the first time we "crossed the pond" as well. It's going to be much easier if you try to digest it in little bits instead of all at once. Guidebooks are going to be a big help, and the net is simply stuffed with good resources.
I'm going to agree that four destinations are too much for the time you have. As Pam pointed out, you could easily lose 2-3 days in transit processes between the U.S. and your first/last locations, and you'd lose the better parts of another three transiting between the other countries: that's 5-6 days of precious time out of your 12-14. Narrowing the field to two would provide you with more time to sightsee, and to really become comfortable with the layouts of your chosen cities, local transport systems, customs and whatnot. It would also allow for a day trip or two, and some flexibility should you need some "down time" or if the weather doesn't exactly cooperate.
Looking at moving your trip to spring or fall is a good idea as well: slightly fewer tourists, a bit lower prices, and more comfortable temperatures in a couple of destinations on your list.
As far as agents go, I'm going to cheerfully duck the shoes the others may pitch at me and tell you that while I wouldn't employ one to completely plan your trip, you may feel more comfortable using one for help with just parts of it - like your flights. I know well-traveled people who do just that because they either just don't like certain pieces of planning, don't have time for it all, or feel that their agent may have access to tools/systems which they don't. A very good, very experienced agent can also sometimes pull some strings for the occasional perk. True, many of them don't have the personal travel experience that I'm betting some of the previous posters on this thread have under their belts so it's good to do some homework before engaging their services.
But we all travel a little differently, and all of us on this thread have the benefit of hindsight so it's OK to do it your way - especially if it helps you to relax and enjoy the planning and anticipation. Can you do this all yourself? Absolutely. Do you want to? That's entirely up to you but the confidence and sense of accomplishment you'll gain by doing as much as possible is well worth the effort!