TS,
first you should be happy ur bf is attractive/wanted

second, if you know very well ur bf loves you very much, and that he shares his sms and whatever the girl did/flirt to him, it shows that he's being honest with you.

maybe from another point of view, ur bf is also proud that another girls likes him. showing u how much attention he has from other girls, maybe he is hinting you in a way to treasure him and love him as much as he loves you.

you can't stop what others can do, you cant just go and give a big slap to the other girl and ask her not to like your bf.. be urself, love your bf like you always did. and most importantly, trust your bf.

don't let the other girl affect you.
in most cases, couples that broke up because they got 'stolen' - also started of quarreling too much out of jealousy.