You are at an age where you need a "best friend", so go with the new one. If you have doubts about how trustworthy she is, don't tell her absolutely everything to start with - trust takes time to build.
That doesn't mean you have to cut off your old one completely, because she will have bf problems sooner or later and will come back to talk to you about them.
If you can, get yourself a Saturday job - it will give you a chance to meet other people from different backgrounds and see how people get on together as work mates.
You gave your friend sensible advice about not being pressurised into sex, but she's on a high with "love" so the chances are she will give in to the boy in the end. When you have a bf, you'll face the same pressures, so stay sensible and only go for sex when you are ready. It may mean that you lose a couple of boyfriends along the way, with all the upset and heartache that goes with it, but in the end you will feel better about yourself for holding off until you are ready.