It happens to all of us, you hit a wall or your body gets use to and basically bore with your work out.
You have to look back and realize all the good you have done and then mentally prepare and physically prepare to get back on course.
Think about it you have lost over 1 lb a week for a year, that is awesome. And your stuck in the 280's - ok lets look at ways to push through your roadblock and get you moving towards the 260's.
1. Diet. You need to go from half decently to all in for a 4 weeks.
Plan your meals - Chicken, spinach, healthy breakfast, bring your lunch - Remove the bad foods completely and reduce or eliminate drinking.
2. Cardio - I have been doing an hour of cardio everyday and on of the personal trainers said mix it up so now I do 20 - 24 minutes of 45 seconds intervals. But I am pushing at the same rate or trying to at 50% level as I am at the 20% level. If you have a trainer check with them first if they recommend this for you.
3. On Weights - Consider doing a weight cardio class (body pump) for two weeks or more. Change it up work with a group, learn some new techniques and then move back to your solo training.
Last - Set a short term goal
The Goal is to blow past your 280 hurdle by the end of May and lose 8 lbs, to achieve this you will focus on diet more, change up your cardio and try some new group classes.
You can do this - Refocus - Take Action and Then Succeed