On your W-4 your would select Single or Married Filing Separately. If you make more than $4,200 (for 2019 - the limit varies year to year), your boyfriend will not be able to claim you next year. For more information on whether your boyfriend can claim you or not see IRS Publication 17 page 25.
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Who is "us?" Did you live with BF the whole year in 2019? Did you have less than $4200 of income in 2019?