Start drinking a large glass of prune juice before bed and that will soften the stool and make it easier.
Im 25 and i had the same symptoms as you. I think it was the student single lifestyle/diet that was making it difficult to go to the bathroom. The weird thing is I had been living on my own since I was 18 but I only started getting fissures when I was 23.
The cream the doctor gave never worked for me.
The beginning of this year I started using an anal lube tube i bought online (easier and cleaner than using my finger) to insert a mix of olive oil and basil oil into my bottom before a bowel movement, after and right before bed. Basil oil has healing properties and is anti-bacterial. It also has a minty feel so its very soothing after a bowel movement.
You can also use coconut oil for insertion.
Try to include more fibre in your diet. More fruit and veg.
I no longer use the oil mix. I only take 1 1000mg tablet of vitamin c + 2 250mg of magnesium every evening after dinner and i have had no fissures for months
Hope that helps. I know its a nightmare!