Should i tell this girl i like her?

There is this girl that i have known for 2 years and we have started talking much more in the last two months and i really like her but she has a boyfriend. We hang out together quite a lot and see each other nearly everyday as we both workout together. This is really playing with my mind because she never leaves me alone she texts me all the time but when i see her with her boyfriend she seems happy. This is after giving me signs that she likes me such as flirting lots over text (talking about sex) always catch her staring at me, laughs at all the unfunny jokes, if were at a party she will always sit by me and follow me if i leave the room and touch me if i walk past her, asking me to help with her homework etc...
She also does talk to me about her boyfriend in both a positive and negative way however when she is in a mood with him i am always the first to know
She also keeps asking me to tell her who i like and this is so confusing because one minute i think she likes me and the next she thinks i like someone else, or could this be to make her want to tell her that i like her. But the thing is i dont get is i think she likes me but then she goes off always and seems happy with her boyfriend. Could this be because she does like me and she doesnt think i like her so she doesnt want to ruin her current relationship?
Laast time were out together she said to me that someone told her that i liked her, and i didnt know what to say. She started to give reasons about why she didnt think i liked her because she has a boyfriend, and she always moans was a few, so i didnt know what to do and said i was talking to her as a mate but i dont know if i should of told her. And also i have been thinking could she of asked this becaouse she wanted me to say yes i like her?
We are good mates and i cant stop thinking about her but i am scared of what she might say when i tell her i like her as it may ruin the friendship we have and i might of missed the ideal chance to tell her that i like her.
What should i do?!