First of all, sex is supposed to be a pleasant, intimate acivity where both you and your partner feels secure, safe and happy to participate.
You mentioned that you are bisexual but are insecure about having sex with both genders. Do you really feel attracted to both genders? Is there anyone (for example your partner) who is (openly or just suggesting tings) pushing you to engage in bisexual activity that you are not comfortable with? If so, please remember that it should always be your decision. Do only what you feel that you want to do. Nobody should tell you how to run your sex life.
You mentioned you really wanted to do this, so I assume that you have an inherent need to have sex with men, but you still need to brake some barriers that have arisen because someone in the past betrayed you.
In this case, find someone you trust and I mean TRUST. Do not seek a one night stand or some guy to have sex with just because you want to try if you can do it or not. Once you have a man you trust and would want to have sex wth, explain to him the situation. Tell him openly that you feel insecure, but at the same time reassure him that it is not something bad that he has done, or something wrong with him. Rememeber this can be difficult for your partner as well, so be mindfull of his emotions.
Once you explain the situation, tell him that you want him to help you overccome this. Make him aware that you need healing and that you can acheive this togeather. Approach this slowly and step-by-step. If you find nudity problematic, then perhaps ask your partner to caress you when you are clothed. Then when you feel comfortable you can take it a step further. If this works out, then through gradual steps you may find yourself enjoying passionate heterosexual sex in no time. This might be a wonderfull voyage of exploration and mutual understanding for both of you.
However, if you believe this is very serious and that you are not making any progress, that you are stuck, that you are unable to overcome said bariers in any way, then it might be best to consult a professional psychologist. There's only so much good advice we can offer, sometimes professional help is needed and there's nothing wrong about seeking it.
