Calling Worksheet name from Cell name

ok guys I got this formula

=SUM('Session 1'!$D$2:$D$9999)

On my summary page

however I need to be able to change the "Session 1" with my adjacent cell

for example my summary page has

Session 1 and here is where I got that formula returning my profit
Session 2
and so on

I tried to drag it down, but I couldn't get the 1 to go to 2 and 3 and 4 etc?

Also part #2 of the question I put 9999 since I do not think my sheet will ever get that long. is their an easier formula to add up as many rows that have data?

Thanks
Matt