I keep thinking negatively about my relationship, like "what if she's cheating" but no matter how much she assures me I'm her only love, I can't stop the negative thoughts. I'm relatively new to relationships. Any advice?

I'll be blunt.

If you don't trust her ... You shouldn't be in a relationship.  It's really that simple.

Just view the thoughts as intrusive and their to make you miserable and tell them "stop."  If you don't allow your mind to go there then the thoughts will lessen. The more power you give to negative thoughts, the more power they'll have. Enjoy your relationship, if you become possessive you'll crush it.  You must learn to enjoy a relationship without becoming too dependent on it for your emotional happiness. Your happiness must come from within. Life's too short to be negative.

Ever hear about the power of suggestion ? Keep up that negative attitude and your fears may very well come to attrition.

I had a bad feeling about my relationship too. Turns out he'd cheated 4 times on me and tried to hide it. I snooped through his phone to find this out otherwise I'd still be clueless. Really hurt ! Sometimes your gut is right, sometimes you could be worrying too much :) in my case, I wasn't

Decide whether or not you should be in a relationship right now. Friendships and relationships are all built on trust. If you are constantly thinking she's cheating on you, then let her go. She doesn't deserve to be treated in a distrustful manner when she has done nothing to warrant it.

If you are new to being in a relationship, then please understand why you want to be with that person. What is it about this person that makes her special?

You also need to know that you cannot and should not control her every action. She is allowed to talk with whomever she wants, text whomever she wants, hang out with her friends - all without you feeling as if she is cheating on you. Just as you have that same freedom.

Ask her to marry you.

Then you'll know for sure.