I have a problem with regard to masturbation. I cannot masturbate without watching porn. Every time I do so I feel that I have joined Satan and everything around me starts to go wrong until a certain point. I'm not sure if I'm letting my mind play tricks on me and then these things really happen. I feel that God forgives me but not immediately. Then again I have a hard time letting things go and forgiving myself.
I like a girl, but that girl doesn't like me. She has a boyfriend of her own, and I used to be jealous of how she treated others and me. After a lot of talking to God and my parents I was able to come to a position where I was able to forget about this girl and not feel jealous. I was able to think of this girl as a friend and then every aspect of how she treated me didn't seem all that bad as I wasn't looking at her as a friend. One night I forgot to pray and all of a sudden I started feeling jealous again. That made me masturbate. Before I masturbated I told God, "God, it's this one time that I didn't pray. I'm sorry and it won't happen again. I want you to get rid of these feelings I have for this girl. I want to concentrate on my studies as I have my main exam in 10 days. I just don't want to have anything to do with this girl. I just want to get over her in my mind." God didn't take her out of my mind immediately. That frustrated me and eventually it led to porn and masturabation. This is my weakness - I am very narrow minded and weak minded too. I want to become more open minded and strong.
After I masturbated I felt all these feeling for this girl dissappear and I felt that I could study again. I felt a little bit of guilt as well. I don't want to masturbate again because I know things will get worse. My question is "Has God forgiven me this time or have I joined Satan?"
There is an assumption that you are making which is false: that you cannot masturbate without looking at pornography. You seem to be a person with strong habits, but strong habits does not mean you have no choice.
Pornography is wrong. It is sinful to watch because it is a depiction of sexual things designed to stir up lust within the viewer. "For this is the will of God, your sanctification: that you should abstain from sexual immorality; that each of you should know how to possess his own vessel in sanctification and honor, not in passion of lust, like the Gentiles who do not know God" (I Thessalonians 4:3-5). See: Lies Pornography Tells Men.
As with any temptation, there is a choice. "No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it" (I Corinthians 10:13). Thus, there is a way to get the relief you need without resorting to pornography.
It appears to me that you tend to use masturbation as a stress reliever. You got jealous and that got you stressed. That led to you wanting to masturbate, even though there are other ways to relieve stress that are just effective. One of the after effects of ejaculating is that a hormone is released that causes the body to relax.
In this particular case you planned in advance to look at pornography and wanted to make a bargain with God that it would be just this one more time. You can't expect to ask God to accept sin. "This is the message which we have heard from Him and declare to you, that God is light and in Him is no darkness at all. If we say that we have fellowship with Him, and walk in darkness, we lie and do not practice the truth" (I John 1:5-6). Notice that even though you wrongly tried to bargain with God, when you did not get the results you wanted, you blamed God. I know it wasn't your intention, but I want you to see clearly the trap you got caught up in. There was no reason to get mad at God when it was you who had unreasonable expectations about this girl.
Just because you sinned, it doesn't necessarily mean you've joined the ranks of Satan's army, especially not permanently. "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us. My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous" (I John 1:8-2:1). This is not an excuse to sin, but since sin does happen, there is a way out of sin for the Christian who refuses to stay in sin.
Therefore, no more excuses regarding pornography. Kick it out of your life.