If you just need a trim and your BF won't do it, why not just go to one of those inexpensive haircut salons and get a trim? If you don't have the funds right now, perhaps you can find a stylist who will be willing to barter for something you have to offer.

If you still think your best option is to trim it yourself, I'd suggest bringing the hair around to the front (across each shoulder) so you can see it; then use a pair of good hair-cutting scissors. Position your fingers near the bottom of your hair (around where you're going to cut) and use your fingers as a guide to follow the cut line (similar to what stylists do, you've seen it done).

I do NOT recommend this, however, as I think it will be too easy to end up with an uneven cut that you might regret later. Spend the money, you'll be glad you did.

P.S. I have some experience with what you're talking about. My hair is as long as yours (or slightly longer); my daughter's hair is down to her thighs (it was to her knees, but she got it trimmed).