I (27M) and my girlfriend (22F) have been together for 2 years now, but the sex between me and her has slowed down tremendously, is that normal?

So first off im glad to still be able to say that i have a sex drive of a 16 year old but im also starting to feel bad and unsure if its me or my girlfriend. So when we first got together we were having sex almost every day like 90% of relationships in the beginning. Well everything was fine and then fast forward to last april she had our now Beautiful daughter who will be 1 and when we met she also already had a 2 month old at the time who also lives with us and will be 2 in april. I also 100% considered her my child being that ive known her since the beginning. But anyways so after we had my daughter which was by C-section, I totally understood that id have to wait 6 weeks and all that good stuff which was fine. Well pretty much after those 6 weeks we went from not just having sex everyday but when we would usually have sex it would just happen to now we barely have sex as in 1-2 times a week or we may go a whole week. Now like i said ill get hard in .5 seconds when we lay in bed and im holding her but majority of the time i go to touch her its an excuse. For example “im tired”, “not tonight”, “u shouldn’t have waited so late” and the list continues. For the times when it does happen I literally have to ask if we can have sex tonight and as much as i love her i try to hold from being upset about it but she can tell when im upset and sometimes causes an argument when i choose to either get on the game or come to bed late or just have a slight attitude. (For my guys yall know that feeling when u think its going to happen and then u get shot down. NIGHT RUINED) Now I honestly love this women and want to marry her but i refues to be married to someone whom i have to ask to have sex with or schedule the sex to happen. Someone please help me with this. Im just confused.