I take metoprolol too but it doesn't do this to me, I'm taking it post op after a aortic valve replacement to keep my heart rate down as I heal (I'm 37, with congenital heart disease).
However, I have heard that before with metoprolol and have read some studies and nothing I have read so far says why people sometimes become depressed after starting a beta blocker. it's wierd, because often times beta blockers in fact help people with depression and/or anxiety problems.
I know my response is of little help, other than to say, I've read of many others having the same problem with the metoprolol. I hope you find something that works for you.
One suggestion... maybe try metoprolol without the ER (extended release).