Eating more to add mass and working out doesn't mean you are going to look like Arnold Schwarzenegger. You can add mass by controlling your diet and doing a moderate amount of weight/strength training without becoming a man mountain.
I understand that body image is a huge issue for a lot of men. It doesn't get the same press as female body issues in the media, but for sure it is pretty common.
Drug centres in England and Wales help people take illegal drugs safely, no questions asked. They are there to stop people who take drugs hurting themselves by doing things like using dirty needles etc... There was an article from the BBC 2 or 3 years ago revealing by far and away the largest group of people visiting these centres for help were young men trying to take steroids. It is a major problem which isn't helped by endless magazines displaying Adonis' who claim to take every supplement under the sun.
You should watch a documentary called Bigger, Faster, Stronger. It's a great documentary about steroid use in the USA and the obsession with male body image. Here is a great clip exposing some of the deceptive practices used by sports photographers to make men look better than they really are. I was shocked to see those "before and after" photos were done in the same day!!:
I do weight training because I enjoy it and because it helps me out now and again in the everyday world. I have a little bit of vanity but by and large I don't worry too much about what I look like. As long as I'm healthy, that's all that matters.
Instead of going straight to the gym, you could try yoga. This will give you a great introduction to bodyweight exercises. Although bodyweight exercises aren't IMO the best way to train to add mass, you can still add a little bit of mass and tone. Additionally yoga has a lot of mental benefits and is generally a very calming practice. You may find that after a few yoga classes you are happier within yourself and don't worry so much how you look. Plus you might be able to talk your girlfriend in to trying yoga too. That way you have a training partner and you can help keep each other motivated.
"Let your food be your medicine, and your medicine be your food"