Firstly there are strict rules in engagement and that means not meeting alone with your to-be wife. Secondly the alternative is to do Nikkah and Rukhsati later if you really want to get married but you are incapable of giving her "full" rights atm.
Why? Do you need a reason really (if you are a muslim)? Because it is zina before nikkah and this would be particularly called fornication. And why again? Because its imitating kafirs. Why yet again? Because engagement is weak relationship, its purpose is to get to know each other and for this purpose its recommended to be short (sexual urges), although there is no set time. And why again? Your abode will be hellfire, and by doing this you are taking your wife-to-be also to hell. If you care for her as you care for her in this world then you won't do this.
Being in love doesn't justify haram thing to be halal.
The punishment of those who die without repenting from zina begins in their grave. In a long hadith, Allah’s Messenger (s.a.w) related a dream in which he saw two men accompanying him (Jibreel and Maalik), showing him how a number of sinners were being punished in al-Barzakh (life between death and Judgement day). The Prophet (s.a.w) said: “…We proceeded until we came across a hole in the ground that resembled a baking pit, narrow at the top and wide at the bottom. Babbling and voices were issuing from it. We looked in and saw naked men and women. Underneath the pit was a raging fire; whenever it flared up, the men and women screamed and rose with it until they almost fell out of the pit. As it subsided, they returned (to the bottom). I said: ‘Who are these?’ They said: ‘…As for the naked men and women who were in the pit, they are men and women who indulge in zina…”(12) In a similar narration Allah’s Messenger (s.a.w) said: “We moved on until I saw people who were awfully swollen, and had the most foul stench, their stench was like that of the sewers. I asked: ‘Who are these?’ They replied: ‘Those are the male and female adulterers.’”(13) As for punishment in the hereafter, Allah Most High says: “And they (i.e. the servants of Allah) do not commit zina - And whoever does this shall meet a full penalty. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace.”(14)
For this reason until you are legally husband and wife it is better to establish fast and prayer because prayer guards again shamefulness and fasting against sexual urges, OR alternatively don't delay nikkah.