I need some tips as I constantly loose the rear in a FBM.

The obvious answer is to add more rear downforce, but if you don't want to do that, have a look at the chart you can find in this post HERE.

What controller do you use (mouse, KB or wheel)?
When exactly do you lose rear: in slow corners, fast corners or something else?
Do you lose rear when your rear tires are cold, well wormed or red hot?
What setup do you use?
Do you have a replay of that happening?

Maybe try learning xrg loosing rear in slow motion

Be more gradual in how you apply the throttle out of corners, and be less aggressive in turn in.
Unless your tires have overheated, there shouldn't be too much rear sliding.

Try lowering rear antiroll and see if that helps, or decreasing rear tire pressure and put more outward camber.

You should make the corners as open as possible.

What controller do you use (mouse, KB or wheel)?Wheel
When exactly do you lose rear: in slow corners, fast corners or something else?Slow corners
Do you lose rear when your rear tires are cold, well wormed or red hot?
What setup do you use? Cold
Do you have a replay of that happening?Nope.

Try 5-4 wings

Quote from N0RTHERNR0GUE :

What controller do you use (mouse, KB or wheel)?Wheel
When exactly do you lose rear: in slow corners, fast corners or something else?Slow corners
Do you lose rear when your rear tires are cold, well wormed or red hot?
What setup do you use? Cold
Do you have a replay of that happening?Nope.

Ok, now we know a bit more
Since you have wheel and analogue throttle, try applying it a bit more gently in those corners.
Increasing rear wings won't help you much in slow corners, where it mostly happens.
Cold blue tires are almost just as bad as red hot.. what happens where they become more green, do you still lose your rear?
Make a replay and attach it here together with the set you use, we could see what exactly happens and help you more.

id start with 4-4 wing 8-6 anti-roll and stiff suspension,i only started to use a wheel not that long ago after using a gamepad for 6+ years and my current best with fbm is now 1.11.96
turn earlier into corners hitting the apex and remembering to only accelerate on exit is also key.

Auto or Manual?

For me, on Auto the FMB likes to spin out due to the jolt from downshifting. Using manual fixes this...

Been on server with OP. He uses wheel, but has a "on and off" throttle style and a very understeery 'pushy' set. Gave him 2 fast sets to try, both capable of 1:11s but OP refered to them as slidey.
He uses a set with the handling almost like the default (scrubs wide when turning with throttle ON).

@OP having seen your driving style and set I'd say your greatest flaw is your driving style. To be quick you have to change it because you can adapt to a fast set to make you quick, but you shouldn't be adapting the set to your style to make it easier. That makes you slower.

Slidey sets are always quicker, imo, sadly. You have to get used to four wheel drifts, oversteer, neutral steering in apex. You also have to carry your braking into corners a touch more, to pitch the weight transfer to the front tires. You also have to apply your throttle mote gradually out of corners, and reduce steering angle as you do so. Plenty of stuff you need to change, that a blazingly fast set cannot do. Hope you take this advice.

Remember, the FBM isn't a BF1 or a F1 2017 car. It isn't point and squirt. You have to man handle the car initially when adapting but after a while you'll realise it takes a lot to string a lap.


Cheers

MicrospecV's advice is spot on, a setup is just one part of the equation with the driving being the other other part. The main thing that i had been told i used to do wrong was braking late and braking while turning as soon as i got the braking points correct i was getting more better times
Its worth listening to any advice from microspecV as i have seen him progress to the laptimes he can get now and believe me he is fast and more important more consistent with fast times compared to myself