You say you have a messenger bag. That should be of a size to fit under the seat. If it isn't, you can: 1) take two bags, including a small bag for your computer that will fit under the seat, or 2) if overhead space is full, take your computer out of the bag, put it under the seat, and check your bag.
(It drives me crazy to see people not use the under-seat space when they have multiple items.)