Your question is about guilt, and guilt comes down to not fulfilling responsibilities. So, let’s consider your potential responsibilities in this scenario. There’s your girlfriend, your guy friend, and yourself. Let’s break it down:
Your girlfriend
Is she your girlfriend or not? Yes, you have history and you live together; but she says you aren’t together anymore, and she leads a separate personal life from you. It sounds to me like your personal life is no longer her business. She is your ex-girlfriend, or friend, or roommate; not your girlfriend. No reason for guilt here.
(Unless, that is, you’ve lied to her about this. Lies are never good. If she asks, you can tell her the truth, or you can tell her it’s none of her business: don’t lie. If you’ve lied, consider apologizing.)
Your guy friend
What kind of friend doesn’t know about his friend’s 9.5-year girlfriend whom he lives with? Have you been leading this guy on; pretending to be something you aren’t? Have you lied? Misled? If so, yes, you have something to feel guilty about, and you ought to confess about the life you lead and your intensions concerning him.
But, if he’s a casual friend, a hook-up, you don’t owe such a person any information about your personal life or past lovers (assuming you aren’t putting them in danger, of an STI for instance; and assuming they don’t ask you something which leads you to tell a lie – and omission of info is a lie). So, to know if you’re guilty here or not, you have to consider which situation is yours.
Yourself
What is it you want out of life? You say you still love your ex-girlfriend: do you want back together with her? Even though you’re on the outs and don’t owe her anything, sleeping with other people right now might not exactly endear her to you: you may be working against yourself, in which case yes, you do have a reason to feel guilty, because you’re screwing yourself over.
On the other hand, are you ready to end things with this girl? Sure, it’s still painful, you still have feelings, but is it over? If so, what do you have to feel guilty about? You’re in a transition period, things are messed up, you’re confused and broken-up about it: that’s what your feelings are about, not guilt. You just need some distance between yourself and the ex. Move out already!