If you make a promise to Allah (SW) you Must implement it with out fail.
If you are sure that you will implement , then make a promise, otherwise it will be sinful to promise and not fulfill.
Seeking and supplicating to Allah (SWT) to make you be with the girl you love is allowed. Allah (SWT) knows the future which we do not know. That is wh we need to seek whatever He knows best for us, not what we think.