Above all else in relationships the most important thing you can do for yourself is listen to your gut. What is it that you're feeling, there are reasons that our body tells us not to do something. There is something known as a "red flag" which is a sign that something is wrong. The list of red flags varies from person to person, as some things are more important to people than others.
A little bit of jealousy is normal, even healthy. But when it gets to the point that everything you do is scrutinized that's an issue. Being extremely possessive and jealous is a sign of disrespect, that person is not concerned about what's best for you--they're just looking out for themselves. Extremely jealous partners tend to curb your appetite for life and make you self-conscious. By breaking you down they're making you conform to what they want you to turn into.
Was this person normal when you started dating and now they're crazy? This is what songs are written about. Riding the emotional roller coaster isn't my idea of a good time, and it's probably not yours either. Is this person impossible to get ahold of, are they possessive about where they've been. Keeping secrets and keeping themselves locked away from you is a sign that something is going on, something that probably shouldn't be happening. If your partner's temper flares out of control be very careful, anger can turn into violence.
Or as I like to call it, too in love with themselves for you to ever compete. The person that's the center of their own universe is never going to find a healthy relationship. Face it, they don't care about your feelings or needs. Like I said before, this is a blatant disrespect, avoid self centered people at all costs.
Ever notice your significant other lying to others or manipulating them? Hopefully it's occurred to you that if they can do it to someone else they can do it to you. Even hiding information is a lie.
There are so many types of red flags out there, but what's important is that you pay attention and pick up on what's wrong. You can't go through life making excuses for someone else and their behavior. While none of us are perfect it's not your fault that your partner acts the way they do. As conscious human beings we're capable of understanding our actions, and most of us understand right from wrong. If you're in a violent relationship, or you are disrespected it's advisable to get out. But, if you're not willing to leave that person yet my best advice is to seek couples counseling.
I hope that these tips have given you something to think about and have gotten you started. Spotting an unhealthy relationship may not be the most fun you've ever had, but it can be one of the best things you do for yourself. If you have a minute I'd like to ask you to visit this video my friend put together. Most of the people I've talked to about relationships in crisis enjoy this website: http://www.themagicofmakingupstore.info I hope that it can do for you what I've seen it do for others.