When I manage to stay sin free for months at a time, I notice I don't get these feelings. I feel like the opposite should happen. The closer I get to the deen, the softer my heart
should be right?
If someone is religiously inclined, immediately after having committed a sin, they will feel remorse.
As time passes, however, this feeling of remorse will dissipate.
This is the same for many things in life. To give an example: you might do a very intense workout and it might be so gruelling that you say to yourself that you won't do it again. But as time goes by, you start to forget just how difficult it was and eventually, you think to yourself that you could do it again.
Once your remorse has diminished, you get consumed by the desire to do it again.
That is I think the psychological aspect to it.
Then, there is the shaytanic aspect. The longer you are away from a sin, he too will spend time diminishing your remorse for it. The closer you get to deen, the harder he will make it for you.
It is a normal uphill struggle. We have to try hard to stop ourselves from sinning and when we do slip up, we pick ourselves up and try again to stop sinning and this struggle should continue. We can never be 100% pure and sinless. It is in our nature to sin and Allah is the most forgiving.