What do you think about me telling my girlfriend that I still love my ex-girlfriend?
Why would you tell your girlfriend that you're still in love with your ex?
Feelings can't be always buried deep down in a heart
Being in a relationship while still having feelings for someone else is unwise. Being direct and honest with your partner can allow them to make whatever choices they feel is correct. If your feelings are too overwhelming, you may want to consider breaking up to avoid hurting your partner.
I'll try my best to do that.
It's great that you are able to make a wise choice for you and your girlfriend. It would be best to avoid hurting her so that you don't have regrets about it in the future.