You may be admitted on any valid visa to the United States, however be prepared to account for your dual immigration intent as "I hope I can stay in USA longer with my american boyfriend who live in upstate NY," definitely raises such presumption.
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No one can predict what will happen at the port of entry, but yes with a valid B-2 visa you will be permitted to enter - if officer concludes that after your stated "purpose of visit" materializes, you will return to your home country. So be prepared to answer questions.
Possibly, but CBP may be concerning about your non immigrant intent since you are traveling with your American boyfriend.
Mr. Shusterman's (former INS Trial Attorney, 1976-82) response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.