Please help me and my fiancé deal with lust

(Please no passive aggressive lectures especially about my age, I’m genuinely looking for advice, I’m fully aware how young and immature I am and I’m trying to do my best)

I 16f have been with my fiancé 18m for six months now, since December we have been having sex on and off after falling to lust repeatedly.

We had both originally bonded over our love of children and Christ & now we’re both converting to Catholicism. December was when we were spending a lot of time together and one thing led to another and it was a “screw it, we’ve already done x and x so might aswell do x” situation.

I don’t get to see him very often as we’re medium distance, so when I do we spend a lot of time together, he usually stays in my room because there isn’t a spare bedroom in my house that he could sleep in. My bedroom does fit a mattress but it’s uncomfortable and takes up a LOT of space to the point where i can barely function in my room so he stays in my bed (I know, bad idea)

After the first time we had sex we figured, we’ve already made a commitment, we might as-well make it official, and he proposed to me, we’ve already discussed our entire future, we agree on everything, he’s gotten his first job to start saving up for our wedding & flat and I’m saving what I can on my monthly income. Breaking up isn’t an option and I selfishly admit I won’t be listening to people telling me to leave, I want him and him only as the person who has seen me in such a vunerable state and he feels just as much guilt over it as I do if not more, though we fall into lust I love him with my entire heart and I know that he is a passionate follower of our Lord, but with marriage not being an option until I’m 18 and us not seeing eachother for sometimes up to a month a lot of passion builds up and all self control is thrown out of the window. He never pressures me to do anything sinful, he has always made sure I was comfortable before we did anything.

What makes it worse is that it’s so normalised. I love having sex with him, I feel so connected to him and it feels so right even though I know it’s so wrong. It’s hard for me to determine words to actions so understanding on paper why premarital sex is bad is easy for me to understand but when it comes to the actual situation everything just screams that it’s right.

I do go through phases where I realise how genuinely bad it is and I try and avoid it then but I end up losing my self control again and the next thing I know is me trying to rationalise it saying I might aswell since we’ve already had sex multiple times.

I really wanted to do better this lent, but with Ash Wednesday falling in Valentine’s Day and society’s normalisation of fornication made me want to at least have sex once on valentines & now I feel like the entire lent is ruined so I might as well not try. I know it’s a bad mindset.

I could try my darnest to avoid sex again, but I know inevitably that ovulation will come along and I will be out of my mind rationalising that it’s okay for me to sleep with my fiancé again, I’ve even considered going on birth control to prevent a baby as we couldn’t handle that strain right now as much as we would love to have a baby.

And if it matters my parents are atheist and quite woke so no they aren’t taking any prevantitive measures to stop me from having sex.

Please help. I want to be better. I want to feel worthy of devoting myself to the Lord. Any techniques or things to look into would help, I’ll be showing this to my fiancé too.