Well done on your progress ?
The only cure to this is more practice, unfortunately. But...
...try and find some easy songs that you can play along to (or sing) which use those open-chord progressions.
Here's one to get you going which is not too fast and where the chord changes are pretty much always on the first beat of each bar (the "1" of "1-2-3-4").
Crowded House - Better Be Home Soon
C - Am - Em - G --> Verse
C - C7 - F - G - C --> Chorus
Bb - D - G
Bb - A7 - D --> Bridge
The chorus and bridge especially will challenge a beginner because of the barred F and Bb chords (if you choose to play them that way) and the odd 7th chord.
There are a few variations on terms of how people play the song, so check out the tabs at
http://www.ultimate-guitar.com/tabs/crowded_house_tabs.htm
You'll find that practicing full songs like these, getting each change nice and fluid, is more interesting and productive than simply going through chord changes all by themselves.
Other than that, there are lots of I-IV-V type songs to choose from, which you can even practice in different keys. Common I-IV-V changes in different keys:
C-F-G (key of C)
A-D-E (key of A)
D-G-A (key of D)
E-A-B (key of E)
F-Bb-C (key of F)
G-C-D (key of G)
Playing through changes in the above order (or variations of them) will be a useful exercise and will be actual changes which you'll use regularly for songs.
