I have a strange relationship with God. I try to be a better person, I try to turn my back on sin and be a good person,
No one can ever be good enough for God. None of us can ever do enough to earn his love, mercy, grace and forgiveness.
If that is your aim - you'll never make it. Sorry, but that's true of everyone who ever lived - with the exception of Jesus.
but I remember past sins and regrets which basically feels like it throws mud on my efforts, I then give up trying and start to switch back to my old ways.
Firstly, if you have confessed your past sins to God, he has forgiven them. If you are in Christ you are a NEW creation - the old has gone, the new has come, 2 Corinthians 5:17.
Secondly, if you are trying to obtain goodness by your own efforts of course you will fail. And then it's easy for the devil to tell us that as we have failed, there's no point in trying again/you've had it. You haven't. If you want to be forgiven the sins of the past - and maybe healed from them, confess them, ask God for forgiveness and for his Holy Spirit to give you strength to live for him.
A while ago I used to want to be a bad person, I used to deliberately be horrible to others and I used to not care because i considered myself one of the Devil's, but due to things in personal life I've realised that's not what I want and I try to change, but when I feel like I've failed if I commit a sin again, or if i feel like God has rejected me, I go into a sort of sulk and start thinking I have more in common with Satan and that's why things aren't working out when I try.
No, you sin because you're human. Christians still live in an imperfect world, and sometimes we may be led astray by the world, the flesh or the devil. But we know that we can be forgiven, start again, do better and have a new life. Don't let your feelings, or the devil, tell you that it's the end when you sin. You may have fallen down, but you can get back up again - maybe with the help of other Christians.
God hasn't rejected you. You used to want to belong to the devil, now you don't - that's God. He made you in his image and wants you to be his child. You have chosen to post in a Christian forum asking for help; that desire to seek God is from him.
Feelings are very fickle - I was a slave to them for several years. But God doesn't stop loving us just because we FEEL like he has.
Some people are of God and some are of The Devil,
ALL people were made by God, in his image.
If some continually sin, it's probably either because they have continually chosen to reject God and his love and their hearts are hardened, or they have never known him, his love and mercy and think there is no hope for them. There is a possibility that some are so mentally ill that they have narcissistic or dangerous personalities or tendencies. Whether sin is actually then their faukt, or something they can't help, is for God to judge.
Some people are of God and some are of The Devil, how do you know exactly which side you belong to? I can't be all bad as I desire a relationship with God,
I don't believe anyone is really ALL bad.
But it's not a matter of badness or goodness. The kindest, gentlest, nicest person in the world is still separated from God - and the only way for them, and us, to be reconciled to God is through Jesus. God loves us so much that he sent Jesus to give his life for us, take our sins upon himself, Matthew 26:28, 2 Corinthians 5:21, and reconcile us to himself. We either accept that we cannot save ourselves, thank God for Jesus, accept him and receive eternal life - or we don't. If we are IN Christ, we have life and every spiritual blessing. But it starts with death - the death of self; admitting that we cannot save ourselves, atone for our sin, make ourselves good enough or, basically, do it our way.
If you desire a relationship with God, fantastic - I believe that is from him and he is already at work in you. So say "yes" to him and his amazing gift, Romans 6:23, thank him for saving you and ask him to fill you with his Spirit - who will help you to stop sinning and will begin to change you into Jesus' image, 2 Corinthians 3:18. Start reading the New Testament and believe what God says and not what your feelings say.
I'm genuinely confused and I worry that I'm kidding myself when I say I want a relationship with God, as I've not been a very nice person in my life,
Some have been in prison, on drugs, members of cults, and yet have found, and accepted, his love and forgiveness.
as he too is technically an outcast for pretty much the same reasons I've always been, namely wanting to do things his own way instead of living upto his expectations.
No. Lucifer was an angel, created by God. But he chose to try to become greater than God; to overthrow him and snatch power. He is not an "outcast", he is an enemy of God and will do everything he can to stop people from believing in/coming to God.
Lucifer did not "fail to live up to God's expectations" - he chose to turn against him.