How can I learn to say "no" without feeling guilty?

Basically, as the title suggests, I have a really hard time saying “no” without feeling guilty about it and, sometimes, sick to my stomach. I’m not sure where this comes from, but I do know it has something to do with a fear of letting people down. For the most part I have no trouble accepting it when someone says “no” to me and I accept it graciously. However, when it comes to me saying “no” to others, it just doesn’t happen easily. It doesn’t bother me to say “yes” to things of little importance so long as everyone involved is happy. However, when it comes to people’s expectations of me, I have a hard time turning them down.

For instance, one of my friends who got engaged a few months ago wants us (meaning she and I, her fiancé and my current flat mate) to get an apartment together when both our leases are up at the end of May. I don’t know how to say this without hurting her feelings or having her get mad at me, but I really don’t want to live with her and her fiancé. Don’t get me wrong, they are really nice people, but I’ve been to their apartments and they’re borderline hoarders. My flat mate and I keep our apartment very clean and organized and we’re mindful of our utility usage. Not only that, but we’re both well-traveled, open-minded and very independent and, unfortunately, the same cannot be said of my friend and her fiancé. In addition, they’re in a different stage of their lives, which is fine, but my flat mate and I are more career-focused right now and I know that, if we were to all move in together, these major differences would not make for a very happy living condition.

I’ve briefly mentioned to my friend that my flat mate and I might be extending our lease and explained to her that, for now, I want to stay where I am because it’s convenient for work. Additionally, I’ve moved around a lot since I was little and I finally feel settled into a place and don’t want to uproot myself yet again – I’m tired of moving. She doesn’t seem to get it and keeps messaging me about looking at apartments. I haven’t seen her in a while, mainly because I’ve been working a lot, but also because I keep avoiding her so I won’t have to tell her no flat-out, which I do realize is very childish, but I just can’t help it…

Any suggestions as to how I should go about this situation? I don’t want to say yes to living together because I know our friendship won’t last very long if we do, but at the same time, I have a strong feeling that, if I say no, she’ll get mad at me. I feel like I’m stuck between a rock and a hard place…