How can I escape fear of being cheated on or left  ?

You have to let go of the past and forgive those who wronged you to allow yourself to have a good future.

You have to take that risk, but if you trusted this person as you should you wouldnt be worrying as you are

It's normal for a couple to talk about what they want in life. At 4 months in for her to tell you she wants YOUR kids? I'd tell her to calm it down. You can both know you want kids and so that's something you have in common for a good year (or two) before deciding that the two of you TOGETHER are ready to start a family.

I've a son who dated a gal for 3 months, she wanted to get married and have a baby so they became engaged... when we reasoned with him to give it a year before the wedding? She broke the engagement immediately and wed someone else 3 months later. She wanted a wedding and a baby. Whatever guy she talked into the instant fulfillment of her fantasy wasn't all that important to her at all. Which was our fear that ended up being entirely true.

Hope this helps. I've never been in a relationship with a girl who loved me as much as your girl loves you, and I’ve never been with a girl who was afraid of losing me. A lot of the girls don't care about their bf's, or even husbands like that, so that makes her rare, since nobody’s real like her. I'm a Christian man who goes to church every Sunday, and I have to say it sounds like the Lord must have blessed you with a wonderful girl you're not supposed to let go of. Whether you believe in him or not, embrace this blessing from the Heavenly father. You’re only holding yourself back out of fear let it go be thankful and embrace this girl or else you’ll loose her because you pushed her away. She loves you she wants you consider yourself a blessed man.

My suggestion is either you let go and love, or not at all because you're only overthinking and stressing versus enjoying the relationship. No one said you have to believe her now, 4 months is super early to know if you truly love someone or not. I would wait to the 6 month-1 year point to reevaluate your relationship.

Based on your description of her it doesn’t sound like she would cheat on you or leave you. 

Realize that it can strain your relationship. If it's going to happen, there's probably nothing you can do to stop it. If she cheat on you, it will say more about her than it says about you. 

Let go of regrets. The benefit of the doubt can be a powerful thing show your partner you trust her and have faith that she’ll be faithful and trustworthy.

Realize your worth. Would you want to be with the kind of person who doesn't treat you like you're worth it? This redditor is right, being alone is better than being in the wrong relationship.

Acknowledge your insecurities. Knowing the difference between insecurities and real threats to the relationship can be a huge help as can giving yourself time to heal if you need it.

Trust your gut. If you can stay balanced and grounded, then you should trust your instincts as much as possible. Sometimes, you just know.

Accept it as a possibility that she is the one for you. Don't assume she's going to cheat on you or leave you. Once you begin to accept that she truly does love you, that she cares about you, and wants to spend her life with you as a possibility, rather than thinking about her breaking your heart, won't take so much room in your thoughts.

Know your boundaries. Boundaries are so important in any relationship. They create mutual respect and can make a relationship so much stronger.

It’s important to be with someone you trust. Sometimes, you just find that person who you can relax with. There's no doubt that cheating can tear a relationship apart but it's also important to remember that anybody can cheat or leave you. That's not to make you think it's inevitable, but it's to allow you to stop obsessing over it so you can try to live your life. Sure, it could  happen and it‘ll be horrible if it did but there’s always a chance that it won’t happen. It’s important to know the signs of cheating, don't let it keep you up at night. 

Ever hear of punctuation? You know, those things that help the reader understand the concepts you're trying to convey? Sheesh. 

You can't ever escape the fear of being cheated on or left. But you can manage it. You can pay attention to what she says and does, and make sure it aligns. You can treat her well and be attentive and kind and respectful. Just be a decent person and expect the same treatment. 

The bottom line is that we cannot control anyone except ourselves. You will have to learn to be satisfied with doing the best you can in every circumstance and that way, even if you LOSE people, (it's not LOOSE), then you'll know that it wasn't meant to be and you can move on to find the person you were meant for. 

You have to learn to trust again sometime. Without trust, what have you? Nobody knows the future, so trust what you have and expect the best.