You don't say "I love you" !!! That would be suggesting other things ... The following or a mixture of them are some of the expressions/words used.: "Best wishes", "Good luck", "Keep in touch", "Don't be a stranger".

Unfortunately, there is no expression in English that is quite like 舍不得你. Greg's suggestions are good. Some other options: "We'll miss you." "We hate to see you leave." "We're sorry to see you go." "We've enjoyed having you here." "It's been great working with you." "Take care" is like 保重 . "Best of luck to you." "We wish you well." (I used "we" in my examples, since the person is leaving a company, but of course you can also say "I," if you want to be more personal.)

Have a safe journey! Come back soon! Bon Voyage! I'll miss you lots and think of you! It was a great joy to have met you.