Guilt comes with our ingrained subconscious knowledge that it's a God-given gift made to be used in the sanctity of marriage. It will never go away as long as you continue. Trust me, it wasn't until I got married after years of fornication that the guilt left and I felt free.
Whether you believe in God or not is irrelevant. God is present in our conscious. He is not the author of confusion and if you're confused or worried, you should stop what your doing now and wait. Good, (the best) things comes to those who wait.
And if your that worried about STD's, go get checked and stop what your doing. It should be a sign that what your doing is dangerous to your health and spiritual sanity and you should take it as such.
As the words of Jesus said when he forgave the prostitute..."Go and sin no more".