If you were never able to completely separate sex and love, then you simply cannot have a FWB.
Even when you can separate both, you can never "guarantee" you won't develop feelings for a FWB, the same way you cannot "guarantee" you won't develop feelings for any person you interact with.
I can separate love and sex, and have had a number of FWBs which have been very successful and enjoyable. But in two occassions I ended up developing feelings for a FWB so had to break the situationship until I recovered from those feelings.
The other way around it also happens: you can meet (and have sex with) the perfect man on paper, and not develop romantic feelings for him. One of my FWBs is like that: stunningly handsome (he actually does part-time modeling), a great guy, sex between us is great, interesting conversations... but the romantic feeling was never there for me. I found it frustrating, because he would have been perfect as a partner and quite fancied me beyond being friends!
Interactions with people always come with the chance of developing into romantic attraction. It does not really matter whether or not you know a relationship would not be possible with them, you can't control who you develop feelings for. With one of the FWBs I developed feelings for, we both knew there was zero chance we could have a relationship. I still fell for him, annoyingly. It happens.
You don't sound like you are the type of person who can enjoy a FWB, OP. I wouldn't recommend it to you.