M.H.
Don't feel bad!! We are going through the EXACT same thing right now! Girl, 5 and Boy 3 are ALWAYS fighting! We separate, put in time out, talk to them, etc. We are about to start taking away whatever it is they are fighting over for the day in addition to a timeout for them. We believe it's just their age and a phase...just something new and exciting to deal with! ;)
Good luck!