dude, you will want to quite soooooo many times, but for me the biggest thing was when i saw all the other recruites, and how they were doing the same things i were, and they were getting through it, so i could to. make sense? you are a family, you won't let eachother fail.
keep a list of reasons you joined the marines with you when you leave for basic, and keep it with you so you can refer to it in hard times. also, press your family friends and loved ones to write to you as much as possible, even every day for your closest ones. that will be a HUGE help.
other than that man, you will want it soooo bad after about the 4th week when everyone is talkin about the cruicible and graduation ceremony. first part is the toughest, but just remember its all for a reason. also the pride and honor you will get, and you'll be part of a brotherhood.
Semper Fidelis man, good luck, God bless.