Megan -
I COMPLETELY understand what you're going through.
Last year when I was really struggling with building up a portfolio. I'd shot at several weddings as a second but didn't feel comfortable using a lot of those images in my portfolio (even though my boss encouraged me to) because they weren't "my" weddings. The weddings I DID get by myself were not ideal for making impressive portfolios. In one, the bride was a beautiful and very, very large girl who I felt wouldn't appeal to the target clientelle I eventually wanted to land (sad but true...I think a lot of women are turned off by big girls and will be more inclined to pick a photographer who has a portfolio full of conventionally pretty people). Another was a pirate-themed wedding, so it was a little...weird...for a portfolio (although it is now one of my most popular albums!). The rest were great, but I felt that my portfolio was lacking some "oomph."
So I looked for a small wedding gown shop in my area and approached them with the offer of giving them a free, all-day photo shoot - they got to keep all the images and could do anything they liked with them EXCEPT fiddle with them in Photoshop - but I would do whatever they wanted to the images in terms of editing. In exchange, I got to use some of their gowns in my shoot.
I found a wedding reception site - a 5-acre landscaped garden - that had just opened for wedding business and needed good photos for their web site. I made the same offer. Let me stage a few weddings on a single day. I'll bring the brides and grooms and flowers (silk arrangements) if you let me shoot on your property for a day. The owner accepted and then it was off to Craigslist to find my models.
I ended up with six "couples" dressed in wedding finery. I spent the whole day taking portraits and having them do fun things like dance, walk through the garden hand-in-hand, etc. It worked out brilliantly. I got many photos to fill out what I felt had been missing in my portfolio. No, I didn't get any shots of the bride getting ready with her girlfriends, the ceremonies, the cake cutting, or the reception...but I did get enough really nice, emotionally impactful portraits to interest actual couples in my services for their real weddings.
You will have to decide for yourself whether you're comfortable with "deceiving" your clientelle by doing something like this. Some photographers would be grossed out by selling fashion photography as wedding photography. But I looked at it this way: I didn't do anything differently with my model couples than I'd do with a real wedding couple. I gave them the same minimal instruction, posed them the same way I'd pose real couples, etc. The only difference was that they weren't really in love.
That is definitely one method of beefing up your portfolio. The icing on the cake is that those two vendors I worked with - the reception site and the dress shop - refer lots of clients to me now.

I hope that at least gives you some ideas on how to boost your portfolio! I was just really honest with these vendors and said, "I'm new to weddings, but not photography. I want to get really serious about weddings and I think we'll be targeting the same clients. Would you like to try working together on this project? It could give us both the boost that we need!"
I think, if you do decide to use this idea, the key is finding new-ish vendors who appear to be in real need of good photography to promote their sites. Look for web sites that are a bit lackin' in the photo department.