At 5 months, you're right, she is tiny, she finds comfort in being close to you. It won't be forever but it is a common age for sleep regression.
Firstly, is she teething? Try calpol 30 min before bedtime. Does she settle better. Suckling could be soothing her gums.
How much is your OH involved? Can he do bath, changed for bed, stories to give you a break? You have to tell him you need 30 minutes, regardless of what happens.
Read No Cry Sleep Solution. Very good suggestions.
Finally. Move furniture our of your room to make way for the cot. This was the best piece of advice I had. I pushed cot up to side of bed and dropped side down, moved bedside cabinet out. We part co slept, so I would transfer her when she was in a deep sleep.
I have to say every 3 hours to feed is normal to me at this age.
I hope you get some sleep soon