I am headed to Europe April 1 for 3 weeks. I am trying to see all I can but don't want to miss something that I shouldn't. I have an itienary and would like you to give me some input on what I should or should not change.

April
1--leave Nashville, TN
2--arrive Geneva
3--Geneva
4--Geneva to Interlaken
5--Interlaken
6--Interlaken to Zermatt
7--Matterhorn
8--Glacier Express
9--st. Moritz to Zurich
10-Zurich and night train to Vienna
11-Vienna
12-Vienna late train to Prague
13-Prague
14-Prague
15-fly to Moscow
16-Moscow
17-fly to St. Petersburg
18-St. Petersburg
19-fly to Copenhagen
20-Copenhagen
21-Copenhagen
22-fly home

I am trying to squeeze in as much as possible. What are the best places to see and where are some decent places to stay. Since I will be by myself some possible hostels or b&b's are not out of the question. Any advice would be muchly appreciated.

Also with all the trains through Switzerland and then the overnight train to Vienna , what would be the best pass to buy. I want to do some activities in Interlaken and Matterhorn. What are the best to do and do they give discounts with the train passes?

seat61.com/switzerland good info for you Chris.
Pass a poor value for me these days I usually train like a local. Sleepers are also kind of not comfy and fairly expensive. I like to day train I see more and do nice hotels
booking.com or more budget eurocheapo.com

Route looks fine to me though...

Go to booking .com for each city ,they are pretty good. The Clarion chain in Scandinavia are good as they include breakfast,afternoon tea and a light supper in most of their room rates.
.When in Prague try to stay in the old city.most of what you want to see will be in the cobbled streets area ,the Jewish cemetery is well worth a visit,it`s heartbreaking.
Walk over the Charles bridge up to the castle and pick a different route back down.
In Moscow Red square,st.Basils,the Kremlin and some of the underground stations are all amazing..you may find the people of Moscow very dour.A guide might be a good idea,as a sole traveller I found it was easier and relaxing as the language/alphabet was a bit beyond me,I only had a short time and wanted to make the most of it.
In St.Petersburg ,Peter and Paul Fortress ,Church of Spilt Blood, Catherine`s palace outside St.Petersburg. And of course The Hermitage.(you could spend months and not see it all)Some of the small cafe`s have excellent food at very good prices,although it can be a hit and miss affair.
Check out Point to Point tickets as well as a pass ,they often work out cheaper.
Enjoy your trip.

I fully understand your desire to squeeze in as much as you can, but I urge you to consider slowing down a bit. You will be spending a LOT of time traveling, and that most definitely WILL eat in to your time to see the magnificent places you have selected. The time people spend in places varies a lot given interests and preferences, but as a rough guideline, I think most people would try to give Vienna several days, Moscow at least 4 or 5 at a bare minimum, and St. Petersburg 4 or 5 days at a bare minimum. I would find having only a day or so in any of these major destinations extremely frustrating. I think that you will find that you are missing so many things that you will wonder why you are rushing around. Just an opinion, but I doubt that I'm the only person who thinks so.

BTW, I've traveled to many of these destinations solo (all but the ones in the Alps) and enjoyed my travels immensely.

Hope that helps!