I haven't told anyone that my ex dumped me months ago
Why have you kept it a secret for this long?
I feel ashamed and I'm so devastated my loss is complete
If you are going through a rough time, you should open up to others. No one can help you if you are hiding the cause of your pain from them.
I'm afraid I'll just hear a bunch of "I told you so's"
If everyone was telling you this was going to be a mistake, perhaps you should consider listening to the advice of others in the future?  Hopefully it can help you avoid making similar mistakes.