I have an almost 5 month old who has rolled tummy to back (hasn’t mastered back to tummy) since 3.5 months and lifts her head and chest quite easily. As of a few weeks ago she will no longer sleep on her back in her crib. If I put her on her tummy she sleeps very happily but due to the risk we are now cosleeping. Should I continue to cosleep or let her sleep on her tummy? I have tried for hours numerous nights to settle her on her back in her crib and can’t handle the lack of sleep sadly.

Most babies will be rolling over by 4- 5 months…...some earlier, some later.     
Most babies seem to roll back to tummy first - but sometimes it is the reverse (as you have found)                       
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side…..& do seem to sleep better when they are on their side or tummy.
It is still recommended to place baby on back & allow them to find their position of comfort.

When you refer to crib do you mean bassinet or cot?
Are you still wrapping/swaddling your baby?
Have you spoken with your Maternal, Child Health Nurse about alternate settling techniques?

What happens if you place her on her back on the floor? does she stay there or does she attempt to roll to tummy?
Are you still doing tummy time when awake? - this also helps to build upper body strength which assists with rolling.

How do you place her for sleep when she is in your bed?
Let me know your responses so I can provide more specific suggestions.

Also have a look at Red Nose information on Co-sleeping & baby rolling
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

https://rednose.org.au/article/Co-sleeping_with_your_baby
https://rednose.org.au/downloads/Co_sleeping_for_families.pdf