You don't have to go it alone at MIT, and you shouldn't. Help is all around you, and you don't even have to know the right place to look. Ask any of the people or offices listed in this section. If they can't help you directly, they'll know who can.
Every MIT student hits a bump somewhere along the road. Don't worry: it doesn't mean that Admissions made a mistake, or that you're just not cut out for [insert challenging field here]. Take a low grade on a quiz or pset as a signal to reassess your learning strategy. Use the resources on this site to check your time management, note-taking, and other study skills—then make some changes.
If you get back two or three psets in a row with low grades, mess up a quiz, or find yourself "just not getting it", call in a coach—a "tutor", an individual who knows the subject well and can help you learn it, too. Visit the Tutoring section for details.
Make the most of your tutoring sessions by working on the material in advance of your appointment. The more specific you can be about your problem areas, the more focused your session will be. Although working through complex and dense material can be a frustrating and slow process, it is an acquired skill that does become easier with time. Don't give up!