Thanks to everyone who spent time to answer.
I've noticed that some of the advice consists of things I've already tried, but that's my fault because I didn't really give any information about what kind of prayer strategies I have unsuccessfully tried. So, I'll provide some background so people might be able to offer more specific advice.
For a long time I believed in the existence of God, but never was really raised with any specific beliefs so I was kind of an “open theist”. I spent a lot of time learning about the major world religions and reading some of their scriptures. Being an American, I was most familiar with Christianity so I started there.
When I was young, I just said the prayers I knew. As I got older, I pretty much did more “open” prayer where I just kind of talked to God. I told him my problems, asked him what things I could work on, asked what advice I need, prayed for other people, and my desires (non-matieral things).
Eventually, though, prayer got harder and harder to do and my belief in a god’s existence was declining. I was very depressed and this was making it worse.
Some Christians told me that God hasn’t heard any of my prayers because I didn’t say them “in the name of Jesus”, so I tried that and my belief still kept declining.
Then they told me that the problem is that I didn’t truly believe in the divinity of Jesus, so even if I say them in the name of Jesus, it won’t help because I don’t actually believe in Christianity specifically. They also said to make sure I pray for forgiveness of sins before I pray.
So, I spent time talking to God and asking him what the correct religion was and asked him to guide me towards the truth. I also started prays by asking for forgiveness for mistakes. I brought up Bible passages like the one about knocking and having the door answered and asked him if he would open the door.
Then I reconsidered the Bible passage where Jesus says not to babble like pagans and say so many words, so I figured maybe God didn’t answer my prayers because he didn’t want me to say so much. So, I just said the Lord’s Prayer and nothing else for a while. Still nothing.
Then I considered that maybe God was communicating and I wasn’t listening. I went back to a more “open” style of prayer where I just try to tell God things, but I kept them short and would just sit in silence for a while trying to pay attention to any thoughts that came to me and nothing seemed to happen.
A went and talked to a Catholic priest and he told me that because I genuinely want to talk to God, it will happen. But, over a few months, I my belief in a god’s existence decreased more and more until my belief was basically nonexistent.
I still tried some prayers, so I asked to be given more faith but my faith was still non-existent. A non-religious person I know sarcastically referenced the biblical passage about how only God can draw people to himself, and said that I probably am just not a member of the elect like Calvinists teach. I never really believed it but it bothered me for some time.
A Buddhist group on my campus gave me basics for meditation, and I researched things on it, but I couldn’t get that to be beneficial at all either.
Some Christians said I might not have been humble enough. I also overheard a Muslim pointing out that people should pray in a prostate position. So, I tried praying on my knees or like some Muslims do by putting their head to the ground.
I gave up for a long time because I literally couldn’t say a prayer. I was always getting the realization that I’m not talking to anyone but my self. Every now and then I would try but it wouldn’t work. Some Christians say that God comes to people, not the other way around. So I assumed that maybe over time, God would come to me. That hasn't happened, though, as far as I can tell.
That was longer than I intended, but that’s what I tried. Maybe now that I've said what I've already tried, people can target things I've done incorrectly or something.