If many of your sexual encounters are casual, or with people you're not serious about, that's likely part of the problem. Consider being in a committed relationship before having sex.

As for working out your issues, a therapist would be your best option. You mentioned being uncomfortable with opening up. The predicament you're currently dealing with sounds uncomfortable. Maybe you should decide what is more taxing - enduring feelings of confusion in regards to your sexuality, and having your past intrude upon how you want to live, or confiding in a professional who is well versed in sexual issues and who has heard it all.