You may have overstepped your abilities a little by going directly to the 250, but you can handle it. You will need a lot of seat time and some patience. When I got back into riding I went to a 125 and really learned how to ride it. I love my YZ 125. I learned to basically hold the throttle open and clutch when appropriate. Well, I just got an '04 YZ 250 and after two days riding I feel really comfortable on it. Learning to ride the 125 to its potential really helped prepare me for the 250.
Now, since you have already chosen the other route, what you need to do is to stay forward on the bike so you can keep the front end down. When hitting jumps, get your head way out over the bars so you are in the correct position to push the front end of the bike down if you get into trouble. Also, learn how to panick brake (remember to pull the clutch in when doing this). You also need to be a little aggressive on the 250 MXer so that you don't get pulled to far back on the bike. When your weight is too far back you aren't controlling the bike, the bike is in control of you. Do you have a gripper seat? If not, you may want to think about getting one (SDG has a nice on for $90). The gripper seat will help you stay forward on the bike. Also, as mentioned above, throttle control is an art on the 250s. But in order to properly control the throttle, you body has to be positioned correctly or it is very easy to fall back on the throttle.
Good luck! You'll get the hang of it.