I dropped straight from D1 to D5 and now I am kind of stuck in D4/D5

Due to illness I was forced to take a break from League for a while and just dropped from that point on.

Now I can't really find a way back up and I don't know what exactly happened that I am struggling this hard right now. It's not necessarily that I am playing bad, but I am frequently losing games I feel like I should be winning and obviously I am doing something wrong then.

Has anyone found himself in a similar situation and might be able to give me some tips, since I am getting slightly desperate.