remember that motives of others are up to God - and that He knows everyone's heart. including yours. so, even if others think you are only being nice because of 'whatever' - you can gain your confidence from Jesus Christ that He also sufferred, was ridiculed, and was assumed to have ulterior motives (as the romans thought He wanted power only - and named him 'king of the jews').
as i read the bible - i take it literally. be kind to those who 'curse' you. that would mean not to curse back. but, to literally be kind. to give water to those who are thirsty (the truth). and, not to be ashamed of living differently than others around you - and trusting that God will work it out. relationships can be funny. you can be in difficulty with someone for years and suddenly the clouds clear. but, it seems that the biblical prescriptive is not to give up. to say - 'well, it isn't working. obviously.' but, to have faith and say 'they are seeing Jesus Christ's actions thru me - and even if they don't believe that i have their interest at heart - Jesus will bless my intentions.' it relieves you of the burden of anger, getting even, mental stress. you're at peace - because you chose the more difficult thing to do. smile and be positive.