PSN Trouble

I got my PSP about 4 years ago when I was 15.
I made an account on PSN via my PSP.
I entered a fake birthdate cuz I was 15.
I just got Resistance Retribution yesterday.
I never needed my PSN till I wanted to play RR multiplayer.
When I tried to sign in to my PSN to play RR multi it said that my password was not valid anymore and I had to change it.
To do that we have to enter our E-mail ID and birthdate.
I couldn't remember the birthdate I entered so i made another account with my actual birthdate.
On my PSP however when I go in account management I need to sign into my old account first and I can't change the password cuz I don't remember the birthdate!
What can I do? I really want to play RR multiplayer.
I checked the Registration E-mail that I got 4 years ago from PSN but the birthdate I entered was not given.
Can I erase the PSP's internal memory so that it doesn't remember about my old account and then I sign in with my new one?
If so then how?

yes, you can. backup everything to the memory stick that you want to keep, take it out, and restore it to factory settings via the settings column in the XMB. (for future reference, use your month and day of your real birthday, and the year of your favorite old muscle car so you will always remember

no problem, glad i could help.