How to change my contract

Dear o2 supporters

I am in a big trouble, and hopefully you can help me.

I had 3contracts with O2, as follows.
My mobile contract - 30 EUR
My wife's mobile - 30 EUR
WLAN - 30 EUR

But O2 granted me 50% off for both mobile contracts, because taking all the services with O2.
So I paid only 60 EUR per month.
30x50% + 30x50% + 30 = 60 EUR

However the WLAN was not fast enough, so I moved to another provider by cancelling the WLAN contract of o2.
Then, the 50% offer of O2 got cacelled, hence the phone contract charge got doubled.
30 + 30 = 60 EUR only for the phone contracts.

I clearly understand that this is correct, but, now I’m paying too much, since my new WLAN provider also charges 60 EUR for me.
That means, now, for both phone contracts from O2 + WLAN from external provider, I need to pay 120 EUR per month.

What I need : 
Either cancel my o2 contracts (because they have unlimited data which I don’t need at all) and go for a cheaper prepaid contracts, or change o2 contracts to a cheaper version.

Note : My current phone contract period is 2 years which ends in 2024.

Can I early cancel, or change my phone contracts, or do you have any suggestion for me, pelase?

Thank you in advance.