Special Forces
Those who quit are Voluntarily Withdrawn (VW) by the course cadre are generally designated NTR or Not-to-Return. This generally ends any opportunity a candidate may have to become a Special Forces soldier. Active Duty military candidates will be returned to their previous units, and IA 18X candidates will be transferred to infantry units as 11B Infantrymen.
Candidates who are "medically dropped," and who are not then medically discharged from the military due to serious injury, are often permitted to "recycle," and to attempt the course again as soon as they are physically able to do so.
Candidates who successfully complete the course but who are "Boarded" and not selected ("Non-Select") are generally given the opportunity to attend selection again in 12 or 24 months. It must be noted, however, that the time window to attend SFAS a second time can be heavily influenced by deployment schedules, as "non-selected" candidates are assigned to infantry units in the meantime.
You would do better to pick a different MOS and concentrate on schooling.