I suppose it all depends on your exact ages and dating/flirting/relationship experience, but I'm getting the big warning bell that says you became interested in her at least partially because she got a boyfriend. Her ability to get a boyfriend means she must have something attractive or appealing about her, either leading you to subconsciously seek it out and/or making you realize that if YOU had her, some other guy might envy YOU, because she now has at least one Boy Seal of Approval.
Alternatively, you might be confusing jealousy with attraction. With a boyfriend, she is probably not showering you with quite as much affection as before (probably some, but not as much.) You are missing that attention... perhaps not her specifically, but the sense that someone was paying attention to you.
Plus there's always a little "want what you can't have." My suggestion is to take a little space from her... don't stop hanging out with her all together, but hang out with her less. Hang out with her and her boyfriend, or her in a group.
As far as "getting over her/finding a girl," start expanding your social circle. Ask yourself what qualities you'd want a girlfriend to have. What similar hobbies. Now, go do those hobbies and keep an eye out for a girl that seems cool that is also doing those hobbies. But DO NOT get in the middle of this relationship. If it's gonna fall apart, let it fall apart on its own accord.