The fact that you can get any type of erection this early is a good sign, and to be encouraged for penile health. You could try doubling or tripling up 5mg Tadalafil and see if erection is better still, as 5mg is a low dose. (You can skip it the next one or two days to get back in sync - you'll still have it in your system as it lasts quite a while.)
As previously said, the fact that you can climax is unrelated to getting an erection, but a handy side effect.