Your friend means well. You have said no, kindly, and the suggestions keep coming. It is hard to disagree with someone who cares about you. Peter also loves Jesus, and still resists what Jesus says about the road ahead.
Read Matthew 16:13–23 as a whole. Peter has confessed Jesus as Messiah. When Jesus then speaks of His coming suffering, death, and resurrection, Peter rebukes Him. The correction that follows concerns Jesus' mission, not an inconvenient social request.
Let the passage question your expectations
It is easy to read this scene looking for the person who needs to stop obstructing us. Try the harder direction first. Where have you decided that following Jesus should protect you from every costly act of love? What part of His teaching do you keep trying to negotiate away?
A sincere confession of faith doesn't make every expectation attached to it right. Peter's affection and conviction are real, yet he needs correction. We can ask for that honesty without borrowing Jesus' severe rebuke as a weapon against someone who disagrees with us.
This is also why the boundary application needs care. A friend making an unwanted suggestion is not Peter opposing the path to the cross. Keep those situations distinct while learning to speak truthfully within a relationship.
Give an honest answer
If you need to decline a request, make the answer clear enough to be understood. If you need to listen to correction, give it a fair hearing. Affection doesn't require agreement, and disagreement doesn't cancel care.
Ask a trustworthy person to help you think when the stakes are high. Costly faithfulness should never be used as a reason to remain in abuse or submit to another person's control.
A question to sit with
Which well-intended suggestion have I treated as an obligation?
You can take this slowly.