You are too young to think of a long term relationship just now. I know you will not agree with me, but believe me, I know. You are in love and so are very emotional just now. Crying will not help. Think rationally.
She did not have an affair, after you got to know her. This happened in the past. If she is 18 now, and this happened earlier, she was too young to even know the consequences of having sex. Yes, you are a virgin and always dreamt of having a wife who is a virgin. If you truly love her, you will forgive her for what she had done in the past.
Your worry about her having contracted a veneral disease due to sex, is genuine. Did she have sex using a condom? Then the chances of any STD are less but you can always rake her to the doctor to get a test done. You said you love her and this is a test of your love. Accompany her to the doctor. Do you have the courage to stand by her? If not, then let her go.
Life is never a bed of roses, though it may seem so when you are young, living on fresh air and your parents money and are in love. Stop pitying yourself. Look for a solution to the problem. If you cannot think of a solution, then let the problem go.