The gap in the traffic is large enough for you to enter. Your are coming out of the proper lane to merge with this traffic. Don't focus on the mirrors for too long. Don't use your mirrors whenever your vehicle is moving forward. Have extra mirrors that show what is ahead of you.
Convex mirrors show a wider area than flat mirrors.
Make everything seem larger. Make everything seem farther away than it really is. Maker everything seem closer than it is. Signal early and signal continuously. Signal 100 feet before you start your turn and cancel the signal as you turn.