Hello AmericanBird, and welcome to the site!
You say that you've just finished a four month study abroad session... was this done through an American university? If so, you will probably be eligable for a BUNAC visa which will allow you to live & work in the UK for up to six months (see
http://www.bunac.org). If you've already been to the UK using the BUNAC scheme, be aware that the rules have recently changed and U.S. students can now go to the UK through BUNAC twice (for six months each time). Unfortunately, it would probably mean a trip back home to organise the BUNAC permit, but their processing times are quite quick.
You say that you've been "doing this for two years now". If you and your boyfriend have been living together for two years, with no more than six months spent apart during those two years, then getting an Unmarried Partner visa may be an option. See
http://ind.homeoffice.gov.uk/ind/en/home/applying/general_caseworking/unmarried_partners.html? . This can be applied for while you are in the UK (although they prefer you to apply from outside the UK).
Hope this helps!
Sara.