I really would appreciate some advice

I'm currently with someone and they have recently shared with me that sex is important to them but I'm still a virgin and have never done that with anyone. I feel ready in certain aspects and I want to do it but I really don't want to go on the pill as I know it would mess with my body. I know condoms break and that's genuinely a large worry of mine. Like i understand that it rare but it happens, but I'm just like in the case it does happen what do I do? Ya ik like take plan b but does that even work? Are there cases where the plan b does not work? I'm honestly a big overthinker and I feel like even if we do it I would stress over it which would cause me to be late on my period. However I have a very normal cycle like I've never been late or irregular. I'm currently a student and I feel like I wouldn't be able to deal with that stress. I'm considering if I just say I don't want to do this but the thing is I do and the thought gets me excited and I'm old and mature enough to do so. I really want some advice and potentially some stress relief. I'm not asking for anyone to be unrealistic and just make me feel better or anything like that but maybe just give me guidance that this is what I would have to do to be precautious. I was thinking of using a condom with the spermicide built into it and also pulling out. I also am using apps to track fertility and stuff so i would avoid those days. Would this be considered being safe? Please don't suggest going on the pill or any hormonal contraception. I don't have a trusted parent or older adult to really speak to about this. Thank you guys sm!