Firstly, only you know if your straight and if you love your gf... we cant answer that...
There are other threads on this forum that cover the same things your feeling, take a look at them, they may help.
Yes you can get bored of sex sometimes with the same person, especially when your young...perhaps you need to spice it up a bit... it doesnt mean you dont love your gf.
Sounds like you are doing better though with the meds and cbt...talk through any concerns with your therapist too.