Stogus, this dilemma is not uncommon. Try to remember that you're not a Christian because you feel like one. You're a Christian because of a commitment you've made to follow Jesus.
Salvation is not a one-time experience, it's a relationship, and therefore it's a process, built on a foundation of trust. Like any relationship, there will be ups and downs. There will be days where you can literally feel the Holy Spirit inside your skin, and some days you feel like God is a million miles away. Every Christian goes through that, due to our imperfect nature.
But a couple of things you said concern me...
I believe that Jesus was without sin, and that he died on a cross for me. I go to church every Sunday and I try to read my Bible when I can.
Every once in awhile I get the feeling that something is missing, that I haven't done enough.
Substitute the word "believe" with the word "trust". Is it still true for you? If you know what happened, but aren't
depending on it, that might be part of the problem. Just because you pray the sinners' prayer doesn't absolve you from having to depend on Jesus for salvation. We have to depend on Him and trust that His sacrifice paid for our sins
continually. Like I said before, salvation is a process, not an event.
The other thing that concerns me is your statement that you might not have "done enough". We cannot
ever 'do enough'. We cannot earn our way into heaven. The Bible says, "For it is by grace you have been saved, through faith, not by works lest any man should boast." If someone (flesh or spirit) is telling you that you're not 'doing enough' to be saved,
you are being lied to.
What I would tell you to do is:
1) Keep on (or start) trusting that Jesus' sacrifice was enough to reconcile you to the Father.
2) Continue to develop your
relationship with God through prayer, Bible study, worship, and service.
