You have suffered a major betrayal kind of ding. I am so sorry.
Your GF has been cheating on you and lying. She's made choices that impact your relationship without consulting you. I do not know if you can trust her again in romantic relationship when she does not choose trustworthy behavior. You sound like you are at your limit and suffering a lot of pain to find yourself there without warning.
I would not trust again if I were in these shoes. I wouldn't want to risk it and be open to another ding of this magnitude.
Things will never
be the same. You cannot "unknow" what you know now.
You CAN choose to forgive without also granting her a do-over chance to make amends. There's different levels of forgiveness. I do strongly suggest moving toward "forgive and let go" even if you do not also grant a second chance because holding on to pain like that can eat you up inside. YOU and your best healths do not need to cling to bitterness forever. It is ok to feel bitter and UGH right now. Draw on this energy to move you forward.
You CAN choose how you behave next.
You COULD choose to move it forward so all people are in a healthier space.
Loving someone does not mean you are blind to their character flaws. Loving someone does not mean you have to throw your OWN best healths under the bus for her pleasure -- mental health, emotional health, physical health, spiritual health. She can continue with her pleasures with you OUT of the line of fire.
I would suggest you Choose YOU. I would suggest you choose your best LONG TERM healths even if you have to endure some more short term suckage ugh to get there.
I would suggest you choose your son.
And I suggest you choose her. Yes. HER too! If you are not together, then she is not cheating on you. That alleviates her of burden also. She was not strong enough to tell you forthright and honest. She has room to grow then. Give her the space to do it in and learn that her choices consequences. You don't have to be vindictive. You could just be simple and clean and honest toward YOU and what YOU need for best self care from a big bruising.
If you move everyone to the place of honesty, and perhaps you both can find a way to heal from this enough to be ok friends and co-parents with her for your son as he continues his growing up.
Asking to be romantic partners as well -- that may be asking to much right now. I don't know. I am hoping her mother skills are better than the romantic partner skills. Sigh.
You do not have to get ugly -- but find out about a fast, quick, clean split.
Perhaps with a mediator? Get you through and OUT of the Time of Suckage without dragging in on.
But you do have to look out for YOUR health. Staying with a chronic lying, cheating person? That's def no good for your mental/emotional health. I can only hope our physical health is not at risk. Get yourself tested.
It is going to suck. It is a Time of Suckage.
But Time? It has the habit of going by no matter what.
In due time I hope you find peace and feel a whole lot better in yourself and not under so much burden. Reach out for that place... a Place of Truly Better and Less Burden. For all of you.
Hang in there. I know it is hard to feel right now.