Too skinny

I just think I'm too skinny. I'm 104 pounds at 21! I'm tall 5 8 1/2 I have basically no butt, and my boobs are small as well. I feel like I'm not good enough for my boyfriend. I noticed he's lost his interest in me. He's not ever wanting sex, and I feel like it's my fault. I've tried so many things to gain weight. I got up to 110 pounds, and lost it right away. I've seen a dr, and they said nothing is wrong. What can I do to put on my weight?