There are probably lots of possible deep psychological reasons why this is happening, but you know I really wonder if you do feel romantic love for your fiancé. What you have said is to describe how much he cares for you and what he puts up with. You also say you love him, but are you attracted to him?

Lack of attraction can be temporary and due to moods and so on, so don't assume that because I ask that question it must mean you aren't physically attracted to him any longer. It's just something to consider. Just because we care for someone and they love us a lot doesn't mean we feel that physical attraction and want to express that love sexually. You are clearly not demonstrating your love for him even if you do feel it. What stops you? How do you feel when he wants your affection? You say you feel suffocated. You need to pay attention to what your body is telling you at the time.

Finally, if you are bipolar are you taking medication? Some medications can reduce sex drive and ability to enjoy sexual pleasure. If this is happening it could account for why you can't respond to his affection and just feel overwhelmed by it. Talking to your doctor about other meds might help.