Most Helpful Girl
Well to be competently honest, the fact that Jasmines best friend was a temptation for you means that you really have no respect for the girl and without respect you have no foundation for a healthy relationship. You're not the shallowest person ever, but it seems pretty clear to me that you don't want to be with her and if this is the case then you shouldn't lead her on because THAT would be shallow. If there's things that you need and want from a relationship and are not currently getting then I would tell you to move on, you owe it to yourself to be happy and there are other available people out there who can provide the things you need. Brenda dose not fall under the 'other available people' category, she's playing you like a game. No girl goes after their own best friends boyfriend without malicious intent.
Most Helpful Guy
This might sound ridiculous and heartless but hear me out.
You're very emotional right now and you need someone to plead a rational case. The fact is, you're young and unsure of what it is you really want. It might be true that you are more attracted to Brenda than you are Jasmine, and this was a timing and placement issue. Jasmine got there first.
Your actions are not out of stupidity. They are derived from a constraint that you are clearly not ready for. That is, being in a monogamous relationship. You may want to be in a monogamous relationship, most of us certainly do. But there's a difference between what we want and what we are ready for. The fact that your connection with Jasmine was not strong enough to override any underlying attraction with Brenda shows that you either don't love Jasmine as much as you should, or you're just not mature enough to handle a monogamous relationship.
As for Jasmine and her hurt feelings, it's normal to want to help her feel better after causing her harm. Empathy is a healthy emotion for a human being to have, just as is the need to want to rectify a broken situation. Don't mistaken this with a strong, immovable love that was unrealized until the damage was done. I think that's where you seem to be the most confused, so it's important to make this distinction.
Another fear that may serve as a false bonding agent with Jasmine is the fear of change. Companionship is something you cherish and being with a girl has many emotional and physical benefits. To lose that is scary. You have to understand that moving on has it's own benefits, you just don't seem them. Opportunities down the road can't be seen unless you're willing to walk. Don't call it a mistake, call it growing up. You'll eventually figure things out.